Spam checker modules can now provide async methods. This is implemented
in a backwards-compatible manner.

Lots of different module apis is not easy to maintain.

Rather than adding yet another ModuleApi(hs, hs.get_auth_handler()) incantation, first add an hs.get_module_api() method and use it where possible.

Add a method to the spam checker to filter the user directory results.

Add documentation for the spam checker.