ENG-5452: Don't log read-only transaction tasks into the task log.

On join, the masters of the new partitions will be logging transaction tasks to
the task log and then replay them later. Since we allow multiple read-only MP
transactions to run concurrently now, it breaks the assumption that only one MP
can be in progress at a time when we do replay.

Read-only transactions don't have to be replayed, so don't bother logging them.
Merge pull request #917 from VoltDB/ENG-5452

ENG-5452: Don't log read-only transaction tasks into the task log.