hammer2 - wire up msg transaction state machine

hammer2 - wire up msg transaction state machine

* Wire up the msg transaction state machine.  Split hammer2_msg into
  hammer2_state and hammer2_msg.  Track states in two RB trees in the
  hammer2_pfsmount structure.

  Handle CREATE, DELETE, ABORT, and REPLY flags.

* Revamp the API.  The adjustments make it less likely that messages can
  be left dangling.

* Compile only, untested, not yet operational.