Refactor Inbound Message: Use an Inbound Backend and only one /sms/momt/ URL

Refs https://github.com/mozilla-services/docs/pull/31
If it can't happen the code should reflect this. Right now it seems that the function that uses the mcc option expect it to have a value - while mnc can be undefined

https://github.com/mozilla-services/msisdn-gateway/blob/sms-backend-wrap-inbound/msisdn-gateway/sms/infos/redis.js#L29