README comments on checksums on page holes.
Avoid tricky race condition recording XLOG_HINT
We copy the buffer before inserting an XLOG_HINT to avoid WAL CRC errors
caused by concurrent hint writes to buffer while share locked. To make this work
we refactor RestoreBackupBlock() to allow an XLOG_HINT to avoid the normal
path for backup blocks, which assumes the underlying buffer is exclusive locked.
Resulting code completely changes layout of XLOG_HINT WAL records, but
this isn't even beta code, so this is a low impact change.
In passing, avoid taking WALInsertLock for full page writes on checksummed
hints, remove related cruft from XLogInsert() and improve xlog_desc record for
XLOG_HINT.

Andres Freund

Bug report by Fujii Masao, testing by Jeff Janes and Jaime Casanova,
review by Jeff Davis and Simon Riggs. Applied with changes from review
and some comment editing.
Skip extraneous locking in XLogCheckBuffer().
Heikki reported comment was wrong, so fixed
code to match the comment: we only need to
take additional locking precautions when we
have a shared lock on the buffer.
Fix calculation of how many segments to retain for wal_keep_segments.

KeepLogSeg function was broken when we switched to use a 64-bit int for the
segment number.

Per report from Jeff Janes.
Minor rewording of README comments