Hot Standby startup with overflowed snapshots

Chris Redekop's recent report of slow startup for Hot Standby has made
me revisit the code there.

Although there isn't a bug, there is a missed opportunity for starting
up faster which could be the source of Chris' annoyance.

The following patch allows a faster startup in some circumstances.

The patch also alters the log levels for messages and gives a single
simple message for this situation. The log will now say

  LOG:  recovery snapshot waiting for non-overflowed snapshot or until
oldest active xid on standby is at least %u (now %u)
  ...multiple times until snapshot non-overflowed or xid reached...

whereas before the first LOG message shown was

  LOG:  consistent state delayed because recovery snapshot incomplete
  and only later, at DEBUG2 do you see
  LOG:  recovery snapshot waiting for %u oldest active xid on standby is %u
  ...multiple times until xid reached...

Comments please.

-- 
 Simon Riggs                   http://www.2ndQuadrant.com/
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