Merge pull request #8300 from jcsp/wip-standby-replay-reorg

mds: Fix standby replay MDS in upgrades and simplify standby replay logic (#15591)

Reviewed-by: Greg Farnum <gfarnum@redhat.com>

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

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

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

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

diff --cc src/mon/MDSMonitor.cc
Simple merge