ENG-4962: Introduce MpTransactionTaskQueue, subclassed from
TransactionTaskQueue.  Use it in place of TransactionTaskQueue in the
MPI.
ENG-4962: SP transaction task queue doesn't need the repair() method.
ENG-4962: Can get rid of this stuff
ENG-4962: Make everything that goes through the MPI flagged as
multi-partition.
ENG-4962: Don't need SP checking in the MpTTQ intake.
ENG-4962: Don't need SP behavior on flush in MpTTQ.  Don't need to
look for fragments of the same TXN ID in MpTTQ.
ENG-4962: Missed this in MP SP purge
ENG-4962: incremental progress
ENG-4962: Okay, think this refactors the queueing
ENG-4962: Important things which can affect state shouldn't be
read-only.
ENG-4962: Think I need a separate path for restart, maybe
ENG-4962: Add the txnID of the completed task as an argument to
TransactionTaskQueue.flush().  Unused by the vanilla TTQ, but I've got
big plans for this guy in the MP version.
ENG-4962: Create read/write sets to track stuff in progress.
Currently, still can only have one transaction of any variety.
ENG-4962: We have separate storage for in-progress txns, don't need to
leave them on the backlog deque anymore.
ENG-4962: Introduce a separate backlog for MP read txns.
ENG-4962: Copy Site to MpRoSite to create the read-only Site thread
for the MPI.  Start deleting unnecessary stuff.
ENG-4962: RO MP Site doesn't need an actually EE, make it go away,
complain if anyone tries to use it.
ENG-4962: RO MP Site doesn't rejoin.  Doesn't need TaskLog.  Doesn't
need DR machinery.
ENG-4962: RO MP site doesn't need to snapshot.  Doesn't need to
initialize an EE.
ENG-4962: Delete more startup things that RO MP Site doesn't need to
do