session: remaining items for MPI sessions

[hzhou]

MPI Sessions has been implemented and we expect all correct programs should work as expected. However, there are a few remaining items to fully meet the semantics.

• Update ref_count in the session structure to account for all communicators created within a session and error check at MPI_Session_finalize
• MPI_Abort only aborts locally in session
• Make MPI_THREAD_SINGLE work with session.
• Fully finalizing and reset global states to a clean slate that can be re-initialized.