pgsql: Put the logic to wait for WAL in standby mode to a separate func

Put the logic to wait for WAL in standby mode to a separate function.

This is just refactoring with no user-visible effect, to make the code more
readable.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/ff8f160bf4322c4294deaa0e64ed26467283d525

Modified Files
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src/backend/access/transam/xlog.c |  429 +++++++++++++++++++------------------
1 files changed, 222 insertions(+), 207 deletions(-)