The header consisted of too many unrelated stuff and macros misuse.
Some stuff is moved into scram.hrl and type_compat.hrl.
All macros have been replaced with the corresponding function calls.
TODO: probably type_compat.hrl is not even needed anymore since
we support only Erlang >= OTP 17.5
1. Add a request handler in ejabberd_http and explain how to configure the http listener so that the challenges can be solved.
2. Make acme configuration optional by providing defaults in ejabberd_acme.
3. Save the CA that the account has been created in so that it creates a new account when connecting to a new CA.
4. Small spec change in acme configuration.
Travis build failed on xref because some functions that I used did not exist in OTP versions 17.5, 18.3
Those functions are: ets:take/2, lists:join/2, erlang:timestamp/0.
