Implement COMMIT_SIBLINGS parameter to allow pre-commit delay to occur

only if at least N other backends currently have open transactions.  This
is not a great deal of intelligence about whether a delay might be
profitable ... but it beats no intelligence at all.  Note that the default
COMMIT_DELAY is still zero --- this new code does nothing unless that
setting is changed.
Also, mark ENABLEFSYNC as a system-wide setting.  It's no longer safe to
allow that to be set per-backend, since we may be relying on some other
backend's fsync to have synced the WAL log.

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

@@ -109,7 +109,8 @@
 #wal_files = 0 # range 0-64
 #wal_debug = 0 # range 0-16
 #commit_delay = 0 # range 0-100000
-#checkpoint_timeout = 300 # range 30-1800
+#commit_siblings = 5 # range 1-1000
+#checkpoint_timeout = 300 # in seconds, range 30-3600
 
 
 #