Add capability to suppress CONTEXT: messages to elog machinery.

Hiding context messages usually is not a good idea - except for rather
verbose debugging/development utensils like LOG_DEBUG. There the
amount of repeated context messages just bloat the log without adding
information.
Convert the PGPROC->lwWaitLink list into a dlist instead of open coding it.

Besides being shorter and much easier to read it changes the logic in
LWLockRelease() to release all shared lockers when waking up any. This
can yield some significant performance improvements - and the fairness
isn't really much worse than before, as we always allowed new shared
lockers to jump the queue.
Improve LWLock scalability.

The old LWLock implementation had the problem that concurrent lock
acquisitions required exclusively acquiring a spinlock. Often that
could lead to acquirers waiting behind the spinlock, even if the
actual LWLock was free.

The new implementation doesn't acquire the spinlock when acquiring the
lock itself. Instead the new atomic operations are used to atomically
manipulate the state. Only the waitqueue, used solely in the slow
path, is still protected by the spinlock. Check lwlock.c's header for
an explanation about the used algorithm.

For some common workloads on larger machines this can yield
significant performance improvements. Particularly in read mostly
workloads.

Reviewed-By: Amit Kapila and Robert Haas
Author: Andres Freund

Discussion: 20130926225545.GB26663@awork2.anarazel.de