Update the DomainRuleChecker module to the new callback and API

[babolivier]

Depends on #110 - I've made this PR target #110's branch to make the diff more readable.

Note to reviewer:

Commits should be reviewable independently. Ideally the 1st commit should be its own PR, but it makes the DomainRuleChecker CI go red so I thought it made more sense to have it in here.

Note to ops:

When deploying a Synapse version including this PR, a Synapse config change is required. All that's needed should be to move the config for the DomainRuleChecker from the spam_checker section to the modules one of the configuration file, since I also had to port the module to the new system. So this means this bit of configuration:

spam_checker:
    - module: synapse.rulecheck.DomainRuleChecker
      config: {...}

Should become:

modules:
    - module: synapse.rulecheck.DomainRuleChecker
      config: {...}

Note that this PR also removes support for the now useless can_only_create_one_to_one_rooms config option, but Synapse or the module won't fail if this option is kept around.

[babolivier]

This turned out to be a mess, so I've reopened #108