Fix bug noted by Bruce: FETCH in an already-aborted transaction block

would crash, due to premature invocation of SetQuerySnapshot().  Clean
up problems with handling of multiple queries by splitting
pg_parse_and_plan into two routines.  The old code would not, for
example, do the right thing with END; SELECT... submitted in one query
string when it had been in transaction abort state, because it'd decide
to skip planning the SELECT before it had executed the END.  New
arrangement is simpler and doesn't force caller to plan if only
parse+rewrite is needed.