Allow a no-wait lock acquisition to succeed in more cases.

Allow a no-wait lock acquisition to succeed in more cases.

We don't determine the position at which a process waiting for a lock
should insert itself into the wait queue until we reach ProcSleep(),
and we may at that point discover that we must insert ourselves ahead
of everyone who wants a conflicting lock, in which case we obtain the
lock immediately. Up until now, a no-wait lock acquisition would fail
in such cases, erroneously claiming that the lock couldn't be obtained
immediately.  Fix that by trying ProcSleep even in the no-wait case.

No back-patch for now, because I'm treating this as an improvement to
the existing no-wait feature. It could instead be argued that it's a
bug fix, on the theory that there should never be any case whatsoever
where no-wait fails to obtain a lock that would have been obtained
immediately without no-wait, but I'm reluctant to interpret the
semantics of no-wait that strictly.

Robert Haas and Jingxian Li

Discussion: http://postgr.es/m/CA+TgmobCH-kMXGVpb0BB-iNMdtcNkTvcZ4JBxDJows3kYM+GDg@mail.gmail.com