Add missing escalus_user_db behaviour
Use escalus_connection:client() instead of ...:transport()

`escalus_connection:transport()` doesn't exist anymore.
Reexport type escalus_config:config/0 as escalus:config/0

Since 1658f58 this type is used but not defined - Dialyzer weeps.
Add TODO about escalus_ejabberd as escalus_user_db
Merge pull request #36 from esl/fix-types-and-userdb

Add missing escalus_user_db behaviour