Bug 1035666 — Add a smsMapping backend mechanism.

This is the first step for Bug 1035666 to let the server find the numberMapping from a backend.
For now the file backend is using the configuration.

I will be working on a Redis implementation as well an HTTP one.

https://bugzilla.mozilla.org/show_bug.cgi?id=1035666
should we return a 200 here? (in case of failure of the hawkId)