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.
Do we need to modify the install instructions in the README?
https://github.com/mozilla/fxa-auth-server#install

> This runs a script scripts/start-local.sh as defined in package.json. This will start up ~~4~~ 3 services, ~~three~~ two of which listen on the following ports (by default):

> - `bin/key_server.js` on port 9000
> - `test/mail_helper.js` on port 9001
> - ~~`./node_modules/fxa-customs-server/bin/customs_server.js` on port 7000~~
> - `bin/notifier.js` (no port)