LP#1729610: extend backlog queue to C apps

LP#1729610: extend backlog queue to C apps

This patch extends the notion of a backlog queue to C apps and
offers the same functionality as the Perl side of the patch series:

- max_backlog_queue configuration setting
- ability to queue messages up to the configured limit
- ability to drop requests that would overflow the backlog
  queue and send status 503 exceptions back to the client.

This patch also adds a new service, opensrf.cslow, that implements
a opensrf.cslow.wait method similar to the Perl opensrf.slooooooow
service.

To test
-------
[1] Set a low max_backlog_queue for opensrf.cslow and a low
    max_children.
[2] Arrange for srfsh to fire off a bunch of opensrf.cslow.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>