Add circular WAL decoding buffer, take II.

Add circular WAL decoding buffer, take II.

Teach xlogreader.c to decode the WAL into a circular buffer.  This will
support optimizations based on looking ahead, to follow in a later
commit.

 * XLogReadRecord() works as before, decoding records one by one, and
   allowing them to be examined via the traditional XLogRecGetXXX()
   macros and certain traditional members like xlogreader->ReadRecPtr.

 * An alternative new interface XLogReadAhead()/XLogNextRecord() is
   added that returns pointers to DecodedXLogRecord objects so that it's
   now possible to look ahead in the WAL stream while replaying.

 * In order to be able to use the new interface effectively while
   streaming data, support is added for the page_read() callback to
   respond to a new nonblocking mode with XLREAD_WOULDBLOCK instead of
   waiting for more data to arrive.

No direct user of the new interface is included in this commit, though
XLogReadRecord() uses it internally.  Existing code doesn't need to
change, except in a few places where it was accessing reader internals
directly and now needs to go through accessor macros.

Reviewed-by: Julien Rouhaud <rjuju123@gmail.com>
Reviewed-by: Tomas Vondra <tomas.vondra@enterprisedb.com>
Reviewed-by: Andres Freund <andres@anarazel.de> (earlier versions)
Discussion: https://postgr.es/m/CA+hUKGJ4VJN8ttxScUFM8dOKX0BrBiboo5uz1cq=AovOddfHpA@mail.gmail.com