Internally, we have supported the option to either wait for all of the
WAL associated with a backup to be archived, or to return immediately.
This option is useful to users of pg_stop_backup() as well, when they
are reading the stop backup record position and checking that the WAL
they need has been archived independently.

This patch adds an additional, optional, argument to pg_stop_backup()
which allows the user to indicate if they wish to wait for the WAL to be
archived or not.  The default matches current behavior, which is to
wait.

Author: David Steele, with some minor changes, doc updates by me.
Reviewed by: Takayuki Tsunakawa, Fujii Masao
Discussion: https://postgr.es/m/758e3fd1-45b4-5e28-75cd-e9e7f93a4c02@pgmasters.net