raft: introduce log storage interface.

This change splits the raftLog.entries array into an in-memory
"unstable" list and a pluggable interface for retrieving entries that
have been persisted to disk. An in-memory implementation of this
interface is provided which behaves the same as the old version;
in a future commit etcdserver could replace the MemoryStorage with
one backed by the WAL.
Require a non-nil Storage parameter in newLog.

Callers must in general have a reference to their Storage objects to
transfer entries from Ready to Storage, so it doesn't make sense to
create a hidden Storage for them.

By explicitly creating Storage objects in tests we can remove a
few casts of raftLog's storage field.
The initial term=0 log entry is now initially unstable.

This entry is now persisted through the normal flow instead of appearing
in the stored log at creation time.  This is how things worked before
the Storage interface was introduced. (see coreos/etcd#1689)
Remove raft.loadEnts and the ents parameter to raft.RestartNode.

The initial entries are now provided via the Storage interface.
Merge remote-tracking branch 'coreos/master' into log-storage-interface

* coreos/master: (27 commits)
  pkg/wait: move wait to pkg/wait
  etcdserver: do not add/remove/update local member to/from sender hub
  etcdserver: not record attributes when add member
  raft: add a test for proposeConfChange
  raft: block Stop() on n.done, support idempotency
  raft: add a test for node proposal
  integration: add increase cluster size test
  integration: remove unnecessary t.Testing argument
  raft: stop the node synchronously
  integration: fix test to propagate NewServer errors
  etcdserver: move peer URLs check to config
  etcdserver: ensure initial-advertise-peer-urls match initial-cluster
  raft: add a test for node.Tick
  raft: add comment string for TestNodeStart
  etcdserver: use member instead of node at etcd level
  raft: nodes return sorted ids
  raft: update unstable when calling stableTo with 0
  *: support updating advertise-peer-url Users might want to update the peerurl of the etcd member in several cases. For example, if the IP address of the physical machine etcd running on is changed, user need to update the adversite-pee-rurl accordingly. This commit makes etcd support updating the advertise-peer-url of its members.
  transport: create a tls listener only if the tlsInfo is not empty and the scheme is HTTPS
  etcdserver: use member pointer for all tests
  ...

Conflicts:
	etcdserver/server.go
	raft/log.go
	raft/log_test.go
	raft/node.go
Merge remote-tracking branch 'coreos/master' into log-storage-interface

* coreos/master:
  etcdserver: add sender tests
  raft: Only call stableTo when we have ready entries or a snapshot.
  etcdserver: add ID() function to the Server interface.
  sender: use RoundTripper instead of Client in sender
raft: remove a guard that is no longer necessary
raft: PR feedback.

Removed Get prefix in method names, added assertions and fixed comments.
Add Storage.Term() method and hide the first entry from other methods.

The first entry in the log is a dummy which is used for matchTerm
but may not have an actual payload. This change permits Storage
implementations to treat this term value specially instead of
storing it as a dummy Entry.

Storage.FirstIndex() no longer includes the term-only entry.

This reverses a recent decision to create entry zero as initially
unstable; Storage implementations are now required to make
Term(0) == 0 and the first unstable entry is now index 1.
stableTo(0) is no longer allowed.
Merge remote-tracking branch 'coreos/master' into log-storage-interface

* coreos/master: (21 commits)
  etcdserver: refactor ValidateClusterAndAssignIDs
  integration: add integration test for remove member
  integration: add test for member restart
  version: bump to alpha.3
  etcdserver: add buffer to the sender queue
  *: gracefully stop etcdserver
  Fix up migration tool, add snapshot migration
  etcd4: migration from v0.4 -> v0.5
  etcdserver: export Member.StoreKey
  etcdserver: recover cluster when receiving newer snapshot
  etcdserver: check and select committed entries to apply
  etcdserver: recover from snapshot before applying requests
  raft: not set applied when restored from snapshot
  sender: support elegant stop
  etcdserver: add StopNotify
  etcdserver: fix TestDoProposalStopped test
  etcdserver: minor cleanup
  etcdserver: validate new node is not registered before in best effort
  etcdserver: fix server.Stop()
  *: print out configuration when necessary
  ...

Conflicts:
	etcdserver/server.go
	etcdserver/server_test.go
	raft/log.go
raft: Rename ErrSnapshotRequired To ErrCompacted.
raft: Integrate snapshots into the raft.Storage interface.

Compaction is now treated as an implementation detail of Storage
implementations; Node.Compact() and related functionality have been
removed. Ready.Snapshot is now used only for incoming snapshots.

A return value has been added to ApplyConfChange to allow applications
to track the node information that must be stored in the snapshot.

raftpb.Snapshot has been split into Snapshot and SnapshotMetadata, to
allow the full snapshot data to be read from disk only when needed.

raft.Storage has new methods Snapshot, ApplySnapshot, HardState, and
SetHardState. The Snapshot and HardState parameters have been removed
from RestartNode() and will now be loaded from Storage instead.
The only remaining difference between StartNode and RestartNode is that
the former bootstraps an initial list of Peers.
Merge remote-tracking branch 'coreos/master' into merge

* coreos/master:
  scripts: build-docker tag and use ENTRYPOINT
  scripts: build-release add etcd-migrate
  create .godir
  raft: optimistically increase the next if the follower is already matched
  raft: add handleHeartbeat handleHeartbeat commits to the commit index in the message. It never decreases the commit index of the raft state machine.
  rafthttp: send takes raft message instead of bytes
  *: add rafthttp pkg into test list
  raft: include commitIndex in heartbeat
  rafthttp: move server stats in raftHandler to etcdserver
  *: etcdhttp.raftHandler -> rafthttp.RaftHandler
  etcdserver: rename sender.go -> sendhub.go
  *: etcdserver.sender -> rafthttp.Sender

Conflicts:
	raft/log.go
	raft/raft_paper_test.go
Re-enable a few tests I had missed.

Fix integration test for the change to log entry zero.

Increase test timeouts since integration tests often take
longer than 10s for me.
Merge remote-tracking branch 'coreos/master' into merge

* coreos/master:
  rafthttp: fix import
  raft: should not decrease match and next when handling out of order msgAppResp
  Fix migration to allow snapshots to have the right IDs
  add snapshotted integration test
  fix test import loop
  fix import loop, add set to types, and fix comments
  etcdserver: autodetect v0.4 WALs and upgrade them to v0.5 automatically
  wal: add a bench for write entry
  rafthttp: add streaming server and client
  dep: use vendored imports in codegangsta/cli
  dep: bump golang.org/x/net/context

Conflicts:
	etcdserver/server.go
	etcdserver/server_test.go
	migrate/snapshot.go
Fix TestSlowNodeRestore
Rename Storage.HardState back to InitialState and include ConfState.

This fixes integration/migration_test.go (and highlights the fact that
we need some more raft-level testing of restoring from snapshots).
raft: use max in log.slice
test: longer test timeout
raft: remove extra empty line in log.go