pgsql: Refactor pg_rewind for more clear decision making.

Refactor pg_rewind for more clear decision making.

Deciding what to do with each file is now a separate step after all the
necessary information has been gathered. It is more clear that way.
Previously, the decision-making was divided between process_source_file()
and process_target_file(), and it was a bit hard to piece together what
the overall rules were.

Reviewed-by: Kyotaro Horiguchi, Soumyadeep Chakraborty
Discussion: https://www.postgresql.org/message-id/0c5b3783-af52-3ee5-f8fa-6e794061f70d%40iki.fi

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/eb00f1d4bf96bdba236bcc089f3ae94db9b7c603

Modified Files
--------------
src/bin/pg_rewind/copy_fetch.c | 14 +-
src/bin/pg_rewind/file_ops.c | 16 +-
src/bin/pg_rewind/filemap.c | 572 +++++++++++++++++++++-------------------
src/bin/pg_rewind/filemap.h | 76 ++++--
src/bin/pg_rewind/libpq_fetch.c | 12 +-
src/bin/pg_rewind/parsexlog.c | 2 +-
src/bin/pg_rewind/pg_rewind.c | 8 +-
7 files changed, 386 insertions(+), 314 deletions(-)