Change pg_last_xlog_receive_location() not to move backwards. That makes

it a lot more useful for determining which standby is most up-to-date,
for example. There was long discussions on whether overwriting existing
existing WAL makes sense to begin with, and whether we should do some more
extensive variable renaming, but this change nevertheless seems quite
uncontroversial.

Fujii Masao, reviewed by Jeff Janes, Robert Haas, Stephen Frost.