- Use host types
- It is best to register hooks last, so it is better not to have
  a dependency, which is started first

The event_pusher one has to run before the inbox one to ensure that
unread_count has not been yet updated by mod_inbox.

mod_inbox_rdbms:get_inbox_unread/4 function always adds 1 to the result

The order can't be easily swapped because of the possibility of having
asynchronous workers for inbox.

- 'push_notifications' still needs the HostType separately
  because the Acc can be just 'ok'

Drop the host type where it can be obtained from the acc

Host type is now obtained from the acc.

Also: don't use the config of the sender's domain (which can be hosted
by a different server) to figure out if the recipient's message
triggers a push notification

- Drop the 'backend' subsection, which provided no extra value
- Do not store backends in ets, the config is enough
- Order backends alphabetically
- Simplify config_metrics logic

- Use maps for opts
- Add a new option with a list of handlers - previously it was
  impossible to set up multiple handlers, which was against the docs.
- Process the path prefix on parsing (only once)
- Use host types

- Use maps for opts
- Use host types
- Use the wpool defaults from mongoose_config_spec

Use host types, update specs for module opts

- Use host types and module opts in a map

- Use maps for opts
- Add a type for exchange opts
- Make the code more straightforward

- Use maps for opts
- Use host types

- Use maps and defaults
- Use a separate helper for the HTTP handlers, because it is an item in
  a list, and thus it doesn't have a path
- Test wpool in a generic way, in the future we can rework other wpool
  tests to do the same
- Check multiple handlers for HTTP

- Do not test changing config on the fly, this is not supported
- Start the full module
- Host type is the same as domain, but in the future we can change this

- Configure event_pusher with maps
- Path should be binary without the leading '/'
- Remove obsolete options that had no effect

- Use opts in maps
- Start module always in init_per_testcase

- Set opts in maps

- Set opts in maps
- Return the correct Acc in process_packet to avoid errors in logs

- Set opts in maps

- Set opts in maps
- Refactor code in a functional way

The backend can be reported from mod_event_pusher_push,
it looks cleaner this way.