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
raft: removed unused func
raft: move poll close to campaign
raft: group configuration related funcs
raft: group getter funcs
Merge pull request #1882 from yichengq/256

etcdserver: correct the log about recovering from snapshot