Log Message:
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Allow read only connections during recovery, known as Hot Standby. 
 
Enabled by recovery_connections = on (default) and forcing archive recovery
using a recovery.conf. Recovery processing now emulates the original
transactions as they are replayed, providing full locking and MVCC behaviour
for read only queries. Recovery must enter consistent state before
connections are allowed, so there is a delay, typically short, before
connections succeed. Replay of recovering transactions can conflict and in
some cases deadlock with queries during recovery; these result in query
cancellation after max_standby_delay seconds have expired. Infrastructure
changes have minor effects on normal running, though introduce four new
types of WAL record. 
 
New test mode "make standbycheck" allows regression tests of
static command behaviour on a standby server while in recovery. Typical and
extreme dynamic behaviours have been checked via code inspection and manual
testing. Few port specific behaviours have been utilised, though primary
testing has been on Linux only so far. 
 
This commit is the basic patch. Additional changes will follow in this
release to enhance some aspects of behaviour, notably improved handling of
conflicts, deadlock detection and query cancellation. Changes to VACUUM FULL
are also required.
 
Simon Riggs, with significant and lengthy review by Heikki Linnakangas,
including streamlined redesign of snapshot creation and two-phase commit.
 
Important contributions from Florian Pflug, Mark Kirkwood, Merlin Moncure,
Greg Stark, Gianni Ciolli, Gabriele Bartolini, Hannu Krosing, Robert Haas,
Tatsuo Ishii, Hiroyuki Yamada plus support and feedback from many other
community members.