V3 of PITR performance improvement for 8.4 (WIP)

This is the V3 of PITR performance improvement (readahead). The
change of the code is as follows:

1) Now readahead is integrated into the core so that it can deal with
sync.rep's log shipping.
2) posix_fadvise() call was integrated with Greg Stark's patch.

Still need some more work for the following:

1) Integrate with sync.rep's wal-receiver part,
2) fix to deal with WAL segment change.

V3 is posted for pre-review purpose and V4 will be posted at the
beginning of January.

Regards;

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Koichi Suzuki