libpqwalreceiver: Convert to libpq-be-fe-helpers.h

libpqwalreceiver: Convert to libpq-be-fe-helpers.h

In contrast to the changes to dblink and postgres_fdw, this does not fix a
bug, as libpqwalreceiver did already process interrupts.

Besides reducing code duplication, the conversion leads to libpqwalreceiver
now using reserving file descriptors for libpq connections. While not strictly
required for the use in walreceiver, we are also using libpqwalreceiver for
logical replication, where it does seem more important.

Even if we eventually decide to backpatch the prior commits, there'd be no
need to backpatch this commit, due to not fixing an active bug.

Reviewed-by: Thomas Munro <[email protected]>
Discussion: https://postgr.es/m/20220925232237[email protected]