Refactor the code implementing standby-mode logic.

It is now easier to see that it's a state machine, making the code easier
to understand overall.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/a068c391ab0c3f2d3487fd3090a769cc5cb19ba9

Modified Files
--------------
src/backend/access/transam/xlog.c |  453 +++++++++++++++++++++++--------------
1 files changed, 285 insertions(+), 168 deletions(-)

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