ENG-4191: Make the masters needed for restart thread-safe.
ENG-4191: Ensure that we always sandwich a restart
CompleteTransactionMessage in between attempts to restart a
transaction.  Interruption of the MpPromoteAlgo could lead to a path
where we didn't do this.
Add CompleteTransactionMessage reception to the Iv2Trace logging.
ENG-4120: Part 2.  Demote log message for unregistered mailbox.  Turns
out it's non-trivial to make the startup code which is generating
these not send the messages to the not-yet-existing ClientInterfaces.
These will still end up in the logfile and any situation which causes
them will be fraught with plenty of other WARNs.
ENG-4185:  Increase the MP deadlock notification timeout to 5
minutes.
ENG-4183: Remove the WARN message when running with IV2 enabled.
Slightly modify the WARN message presented if legacy mode is chosen.