ENG-5711: WIP code to run mixed versions of VoltDB in a cluster.
Merge branch 'ENG-5712' into ENG-5799
Merge remote-tracking branch 'origin/ENG-5712' into ENG-5711
ENG-5799 clean up volt compiler error messages.
Merge remote-tracking branch 'origin/master' into ENG-5711
ENG-5799 Got rid of a pointless error message.
ENG-5711: Make Hotfix work. Seems to now.
Added tests.
Merge remote-tracking branch 'origin/master' into ENG-5711

Conflicts:
	src/frontend/org/voltdb/compiler/DDLCompiler.java
ENG-5711: Make sure we can run with version strings that have alpha in them.
Merge remote-tracking branch 'origin/ENG-5712' into ENG-5711
Merge remote-tracking branch 'origin/ENG-5712' into ENG-5711

Conflicts:
	src/frontend/org/voltdb/utils/CatalogUtil.java
	tests/frontend/org/voltdb/utils/TestCatalogUtil.java
Merge remote-tracking branch 'origin/master' into ENG-5711
ENG-5711: Self-reviewed cleanup.
For ENG-5927, don't log in client when subscribe procedure isn't found in pre 4.1 clusters
Merge remote-tracking branch 'origin/master' into ENG-5711
ENG-5711: Taking review feedback.
For ENG-5927, add missing return
ENG-5916: Command log replay end of log handling changes.

Simplified ReplaySequencer by removing EOL message for each partition, only the
MPI sends EOL message to SPIs now, which indicates EOL of all partitions.

Add poison flag to MpReplayAckMessage to indicate MPs with missing
sentinels.
ENG-5711: Taking scooper's feedback.
ENG-5711: Fix test bug.