This gets rid of XLByteLT, XLByteLE, XLByteEQ and XLByteAdvance.
These were useful for brevity when XLogRecPtrs were split in
xlogid/xrecoff; but now that they are simple uint64's, they are just
clutter.  The only downside to making this change would be ease of
backporting patches, but that has been negated by other substantive
changes to the involved code anyway.  The clarity of simpler expressions
makes the change worthwhile.

Most of the changes are mechanical, but in a couple of places, the patch
author chose to invert the operator sense, making the code flow more
logical (and more in line with preceding comments).

Author: Andres Freund
Eyeballed by Dimitri Fontaine and Alvaro Herrera