TODO LIST:

TODO LIST:
 - AssignTransactionId() needs to WAL log sooner, before actually getting
   the new XID
 - AssignTransactionId() should also update the SnapArray before getting
   the new XID, even in normal operation, if there are too many cached subxids
 - Rewrite TransactionIdIsInProgress().
 - Compute global_xmin properly.
 - Review residual references to GetSnapshotData().
 - ProcArray code should not take SnapArrayLock.