Extend Cluster.pm's background_psql() to be able to start asynchronously

Extend Cluster.pm's background_psql() to be able to start asynchronously

This commit extends the constructor routine of BackgroundPsql.pm with a
new "wait" parameter.  If set to 0, the routine returns without waiting
for psql to start, ready to consume input.

background_psql() in Cluster.pm gains the same "wait" parameter.  The
default behavior is still to wait for psql to start.  It becomes now
possible to not wait, giving to TAP scripts the possibility to perform
actions between a BackgroundPsql startup and its wait_connect() call.

Author: Jacob Champion
Discussion: https://postgr.es/m/CAOYmi+=60deN20WDyCoHCiecgivJxr=98s7s7-C8SkXwrCfHXg@mail.gmail.com