Up to this point, code supported STM-like interface for local data.
This commit adds a lot of the supporting machinery for interactions
*between* endpoints:

    * Added a message to request sequence blocks to be executed on
      partner endpoints

    * Added code paths for committing and backing out commits across
      endpoints.  (Still an opportunity for deadlock though.)

    * Etc.

None of this code is really tested.  The next several commits will add
more commit, etc. mechanisms to this code as well as test it.