Add lock_timeout configuration parameter.

This GUC allows limiting the time spent waiting to acquire any one
heavyweight lock.

In support of this, improve the recently-added timeout infrastructure
to permit efficiently enabling or disabling multiple timeouts at once.
That reduces the performance hit from turning on lock_timeout, though
it's still not zero.

Zoltán Böszörményi, reviewed by Tom Lane,
Stephen Frost, and Hari Babu

src/backend/utils/misc/postgresql.conf.sample

@@ -489,6 +489,7 @@
 #default_transaction_deferrable = off
 #session_replication_role = 'origin'
 #statement_timeout = 0			# in milliseconds, 0 is disabled
+#lock_timeout = 0			# in milliseconds, 0 is disabled
 #vacuum_freeze_min_age = 50000000
 #vacuum_freeze_table_age = 150000000
 #bytea_output = 'hex'			# hex, escape