The Great Refactor (part I): make SMS handling actually manageable

- Messages now get shoved into the DB before anything else, meaning
  we never drop 'em on the floor. (fixes #14)
- SMS->Matrix handling has been completely reworked. Now, we create
  a UserManager (through the RecipientFactory) for each phone number,
  which can be prodded in order to make it get messages from the DB
  and deliver them. (fixes #13)
- Message delivery is now actually failure-tolerant as well!
  We do an exponential backoff for failures to deliver.
- We now do regular CMGLs if configured, and also if an earlier one
  failed to work completely. This can be configured with the cmgl_secs
  value in the configuration. (fixes #8)
- Modems without CNMI support now work with the above polling mechanism.
- We now use newer versions of glitch-in-the-matrix and huawei-modem.