FOR [SHARE|UPDATE] NOWAIT may still block in EvalPlanQualFetch

FOR SHARE|UPDATE NOWAIT will still block if they have to follow a ctid
chain because the call to EvalPlanQualFetch doesn't take a param for
noWait, so it doesn't know not to block if the updated row can't be locked.

The attached patch against master includes an isolationtester spec to
demonstrate this issue and a proposed fix. Builds with the fix applied
pass "make check" and isolationtester "make installcheck".

To reach this point you need to apply the patch in
http://www.postgresql.org/message-id/51FB4305.3070600@2ndquadrant.com
first, otherwise you'll get stuck there and won't touch the code changed
in this patch.

--
 Craig Ringer                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services