Ensure that WAL pages skipped by a forced WAL switch are zero-filled.

Ensure that WAL pages skipped by a forced WAL switch are zero-filled.

In the previous coding, skipped pages were mostly zeroes, but they still
had valid WAL page headers.  That makes them very much less compressible
than an unbroken string of zeroes would be --- about 10X worse for bzip2
compression, for instance.  We don't need those headers, so tweak the logic
so that we zero them out.

Chapman Flack, reviewed by Daniel Gustafsson

Discussion: https://postgr.es/m/579297F8.7020107@anastigmatix.net