LP#1729610: return new OpenSRF status if backlog queue fills up

LP#1729610: return new OpenSRF status if backlog queue fills up

This patch teaches Perl services how to return a new OpenSRF status,
OSRF_STATUS_SERVICEUNAVAILABLE (code 503) if the backlog queue
for a service gets full.

To test
-------
[1] Set a low max_backlog_queue for opensrf.sloooow and a low
    max_children.
[2] Arrange for srfsh to fire off a bunch of opensrf.sloooow.wait
    requests.
[3] Verify that requests that come in after the backlog queue fills
    up immediately get 503 exceptions.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>