Pre-apply the bootstrapping ConfChange entries.

This eliminates the need to fake an ApplyConfChange call before Campaign
in tests.

Fixes #1856.
raft: Ignore redundant addNode calls.

This avoids clobbering any state when bootstrapping entries are
applied twice.
rafthttp: increase sender buffer size

The buffer size is set big enough to buffer all messages generated in
one second as a follower in good path.
Merge pull request #1878 from yichengq/253

rafthttp: increase sender buffer size
Merge pull request #1874 from bdarnell/bootstrap-apply

Pre-apply the bootstrapping ConfChange entries.
doc: add doc for changing peer urls
doc: refactor other_apis.md
doc: http status for other_apis.md
Merge pull request #1879 from xiang90/peer_url_doc

doc: add doc for changing peer urls
doc: add doc for member migration
Merge pull request #1883 from xiang90/member_migration

doc: add doc for member migration
doc: add a link to the peer url section in other_apis.go
doc: fix typo
Merge pull request #1885 from xiang90/doc

Doc
raft: print out log when creating a newraft
Merge pull request #1886 from xiang90/raft_log

raft: print out log when creating a newraft
etcdserver: correct the log about recovering from snapshot
etcdserver: check recovery error when new server
raft: refactoring logging
Merge pull request #1887 from xiang90/raft_logging

raft: refactoring logging