ENG-6114: Set the involved partitions when an MP is being restarted.

Refactored the FragmentTaskMessage so that the InitiateTaskMessage and the
involved partitions set have to go in together. These two things are both set in
the first fragment for command logging.
ENG-6114: Update method comment for FragmentTaskMessage.setStateForDurability().
Merge pull request #1273 from VoltDB/ENG-6114

ENG-6114: Set the involved partitions when an MP is being restarted.