ENG-4834: Fix MP restart with sync command log run out of order.

With synchronous command logging, a restarted MP txn could be offered to the
transaction task queue before the original first fragment was marked
durable. As a consequence, The original first fragment could be executed after
the restarted MP txn. This led to MP deadlocks and fragments being run twice.
ENG-4834: Change queue add to offer for better readability.
Merge pull request #582 from VoltDB/mprestart

ENG-4834: Fix MP restart with synchronous command logging out of order issue