mon/MDSMonitor: do not pointlessly kill standbys that are incompatible with current CompatSet
During a rolling upgrade, standbys may suicide once the CompatSet for the FSMap is updated. This needlessly complicates the rolling upgrade process by requiring all standby daemons to be stopped before upgrading rank 0. We do not need to worry about an incompatible standby taking over for a file system because it will still do its compatibility check when promoted to up:replay (for the case where a higher version MDS is promoted, updates the compatset, and then fails allowing an older MDS to takeover).
Also, the compatset of each file system is updated whenever any MDS reports a new compatset. This also complicates the rolling upgrade because an upgrade of any MDS will kill rank 0 for all file systems. Only upgrade the compatset of the MDSMap if one of the ranks upgrades.