pgsql-server: Handle impending sinval queue overflow by means of a

Log Message:
-----------
Handle impending sinval queue overflow by means of a separate signal
(SIGUSR1, which we have not been using recently) instead of piggybacking
on SIGUSR2-driven NOTIFY processing. This has several good results:
the processing needed to drain the sinval queue is a lot less than the
processing needed to answer a NOTIFY; there's less contention since we
don't have a bunch of backends all trying to acquire exclusive lock on
pg_listener; backends that are sitting inside a transaction block can
still drain the queue, whereas NOTIFY processing can't run if there's
an open transaction block. (This last is a fairly serious issue that
I don't think we ever recognized before --- with clients like JDBC that
tend to sit with open transaction blocks, the sinval queue draining
mechanism never really worked as intended, probably resulting in a lot
of useless cache-reset overhead.) This is the last of several proposed
changes in response to Philip Warner's recent report of sinval-induced
performance problems.

Modified Files:
--------------
pgsql-server/src/backend/commands:
async.c (r1.110 -> r1.111)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/commands/async.c.diff?r1=1.110&r2=1.111)
pgsql-server/src/backend/postmaster:
postmaster.c (r1.393 -> r1.394)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/postmaster/postmaster.c.diff?r1=1.393&r2=1.394)
pgsql-server/src/backend/storage/ipc:
sinval.c (r1.62 -> r1.63)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/storage/ipc/sinval.c.diff?r1=1.62&r2=1.63)
sinvaladt.c (r1.54 -> r1.55)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/storage/ipc/sinvaladt.c.diff?r1=1.54&r2=1.55)
pgsql-server/src/backend/tcop:
postgres.c (r1.413 -> r1.414)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/tcop/postgres.c.diff?r1=1.413&r2=1.414)
pgsql-server/src/include/commands:
async.h (r1.23 -> r1.24)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/commands/async.h.diff?r1=1.23&r2=1.24)
pgsql-server/src/include/storage:
pmsignal.h (r1.6 -> r1.7)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/storage/pmsignal.h.diff?r1=1.6&r2=1.7)
sinval.h (r1.33 -> r1.34)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/storage/sinval.h.diff?r1=1.33&r2=1.34)