wal: add a doc file
wal: add more explanation to wal filename format
wal: wal.OpenFromIndex -> wal.OpenAtIndex

The first entry read from the wal should be the index provided. This
name makes that more clear.
etcdserver/etcdhttp: split out peers and add tests
raft: test node step
Merge pull request #1054 from philips/add-wal-doc

wal: add a doc file
etcdserver: switch to proper int64Slice for sorting
raft: comments for TestNodeStep
etcdserver: use int64slice directly
Merge pull request #1060 from jonboulle/peers

etcdserver/etcdhttp: split out peers and add tests
Merge pull request #1059 from coreos/node_test

test node step
wal: change wal filename format

Make raftIndex section to be expected raftIndex of next entry.

It makes filename more intuitive and straight-forward.

The commit also adds comments for filename format.
Merge pull request #1045 from unihorn/119

wal: change wal filename format
raftLog: compact applied entries only

compact MUST happen on entries that have been applied, or
1. it may screw up the log by setting wrong commitIndex
2. discard unapplied entries
Merge pull request #1049 from unihorn/120

raftLog: enhance check in compact
etcdserver: introduce Server interface

This changes etcdserver.Server to an interface, with the former Server
(now "server") becoming the canonical/production implementation.
This will facilitate better testing of the http server et al with mock
implementations of the interface.
It also more clearly defines the boundary for users of the Server.
A NewServer convenience function is added to create the Server with some
default/production values.