Be more tense about not creating tuplestores with randomAccess = true unless

Be more tense about not creating tuplestores with randomAccess = true unless
backwards scan could actually happen.  In particular, pass a flag to
materialize-mode SRFs that tells them whether they need to require random
access.  In passing, also suppress unneeded backward-scan overhead for a
Portal's holdStore tuplestore.  Per my proposal about reducing I/O costs for
tuplestores.