Merge branch 'stable-3.0' into stable-3.1
* stable-3.0:
Clarify the limitations of gerrit+ssh in replication.config
PushOne: Don't log refs to push at ERROR level
PushOne: Remove redundant 'throws' declarations
Destination: Further improve the debug logs when not pushing
Improve logging of why a project or ref is not replicated
ReplicationQueue: Add handling of null ReplicationTasksStorage.ReplicateRefUpdate
Destination: Extract repeated string to a constant
ReplicationQueue: Migrate to Flogger
Change-Id: Ib78ceca4d6f40f31bb80fca188228ae6aa263c23