Support an optional asynchronous commit mode, in which we don't flush WAL

before reporting a transaction committed.  Data consistency is still
guaranteed (unlike setting fsync = off), but a crash may lose the effects
of the last few transactions.  Patch by Simon, some editorialization by Tom.

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

@@ -149,6 +149,7 @@
 # - Settings -
 
 #fsync = on				# turns forced synchronization on or off
+#synchronous_commit = on		# immediate fsync at commit
 #wal_sync_method = fsync		# the default is the first option 
 					# supported by the operating system:
 					#   open_datasync