Replace max_standby_delay with two parameters, max_standby_archive_delay and

max_standby_streaming_delay, and revise the implementation to avoid assuming
that timestamps found in WAL records can meaningfully be compared to clock
time on the standby server.  Instead, the delay limits are compared to the
elapsed time since we last obtained a new WAL segment from archive or since
we were last "caught up" to WAL data arriving via streaming replication.
This avoids problems with clock skew between primary and standby, as well
as other corner cases that the original coding would misbehave in, such
as the primary server having significant idle time between transactions.
Per my complaint some time ago and considerable ensuing discussion.

Do some desultory editing on the hot standby documentation, too.

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

@@ -186,15 +186,19 @@
 # - Streaming Replication -
 
 #max_wal_senders = 0		# max number of walsender processes
-#wal_sender_delay = 200ms	# 1-10000 milliseconds
+#wal_sender_delay = 200ms	# walsender cycle time, 1-10000 milliseconds
 #wal_keep_segments = 0		# in logfile segments, 16MB each; 0 disables
 
 # - Standby Servers -
 
-#hot_standby = off		# allows queries during recovery
-#max_standby_delay = 30s	# max acceptable lag to allow queries to
-				# complete without conflict; -1 means forever
-#vacuum_defer_cleanup_age = 0	# num transactions by which cleanup is deferred
+#hot_standby = off			# "on" allows queries during recovery
+#max_standby_archive_delay = 30s	# max delay before canceling queries
+					# when reading WAL from archive;
+					# -1 allows indefinite delay
+#max_standby_streaming_delay = 30s	# max delay before canceling queries
+					# when reading streaming WAL;
+					# -1 allows indefinite delay
+#vacuum_defer_cleanup_age = 0		# number of transactions by which cleanup is deferred
 
 
 #------------------------------------------------------------------------------