Merge pull request #1793 from ceph/wip-multimds

Speed up several cross-MDS operations by reducing the number of two-phase commit disk accesses we have to go through.

Reviewed-by: Greg Farnum <greg@inktank.com>

diff --cc src/mds/Locker.cc
Simple merge

diff --cc src/mds/MDCache.cc
Simple merge

diff --cc src/mds/Mutation.h
Simple merge

diff --cc src/mds/Server.cc
Simple merge

diff --cc src/messages/MMDSSlaveRequest.h
Simple merge