Remove the CheckpointStartLock in favor of having backends show whether they

Remove the CheckpointStartLock in favor of having backends show whether they
are in their commit critical sections via flags in the ProcArray.  Checkpoint
can watch the ProcArray to determine when it's safe to proceed.  This is
a considerably better solution to the original problem of race conditions
between checkpoint and transaction commit: it speeds up commit, since there's
one less lock to fool with, and it prevents the problem of checkpoint being
delayed indefinitely when there's a constant flow of commits.  Heikki, with
some kibitzing from Tom.