Use os.File.Sync.
Minor refactor of AddPeer/RemovePeer.
Use github.com/coreos/go-raft import path

Rebase this patch on top of the upstream branch for keeping the coreos
go-raft paths. This is needed for the recently added protobuf
sub-package.

    git pull --rebase origin upstream
chore(server): spelling error

s/connectiongString/connectionString/
fix takesnapshot
Merge branch 'upstream' of github.com:coreos/go-raft into upstream
sort the commitindex of peers in descending order