Re: Improve read_local_xlog_page_guts by replacing polling with latch-based waiting

Hi,

Some changes in v3:
1) Update the note of xlogwait.c to reflect the extending of its use
for flush waiting and internal use for both flush and replay waiting.
2) Update the comment above logical_read_xlog_page which describes the
prior-change behavior of read_local_xlog_page.

Best,
Xuneng