Make a couple of small changes to the tuplestore API, for the benefit of the

upcoming window-functions patch.  First, tuplestore_trim is now an
exported function that must be explicitly invoked by callers at
appropriate times, rather than something that tuplestore tries to do
behind the scenes.  Second, a read pointer that is marked as allowing
backward scan no longer prevents truncation.  This means that a read pointer
marked as having BACKWARD but not REWIND capability can only safely read
backwards as far as the oldest other read pointer.  (The expected use pattern
for this involves having another read pointer that serves as the truncation
fencepost.)