basic_archive: Allow archive directory to be missing at startup.

basic_archive: Allow archive directory to be missing at startup.

Presently, the GUC check hook for basic_archive.archive_directory
checks that the specified directory exists.  Consequently, if the
directory does not exist at server startup, archiving will be stuck
indefinitely, even if it appears later.  To fix, remove this check
from the hook so that archiving will resume automatically once the
directory is present.  basic_archive must already be prepared to
deal with the directory disappearing at any time, so no additional
special handling is required.

Reported-by: Олег Самойлов <splarv@ya.ru>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Sergei Kornilov <sk@zsrv.org>
Discussion: https://postgr.es/m/73271769675212%40mail.yandex.ru
Backpatch-through: 15