Biggish refactor of dependencies

Got rid of all custom git dependencies and obliterated mysql and heap db.

The new fxa-auth-db-mem dev dep hosts a memory db over the httpdb
api.
This is a bit hacky, but now that the db-server is in a separate process we can test this with any backend. The hacky bit is that TestServer tries to run a mem backend for itself so I just kind of paint over those errors and pretend everything is ok. Also, now this code is pretty hairy, but we can come back to it.