Before the fix every 2PC commit/abort leaked a file descriptor. As the
files are opened using BasicOpenFile(), that quickly leads to the
backend running out of file descriptors.

Once enough 2PC abort/commit have caused enough FDs to leak, any IO
in the backend will fail with "Too many open files", as
BasicOpenFilePerm() will have triggered all open files known to fd.c
to be closed.

The leak causing the problem at hand is a consequence of 0dc8ead4,
but is only exascerbated by it. Previously most XLogPageReadCB
callbacks used static variables to cache one open file, but after the
commit the cache is private to each XLogReader instance. There never
was infrastructure to close FDs at the time of XLogReaderFree, but the
way XLogReader was used limited the leak to one FD.

This commit just closes the during XLogReaderFree() if the FD is
stored in XLogReaderState.seg.ws_segno. This may not be the way to
solve this medium/long term, but at least unbreaks 2PC.

Discussion: https://postgr.es/m/20200406025651.fpzdb5yyb7qyhqko@alap3.anarazel.de