Add traffic watcher services for the on_start/on_stop callbacks (#27)

[evdokimovs]

Part of #27

Required for #91

Synopsis

#91 has become too big. And it was decided to break it into several PRs.

This part of #91 is responsible for the RTCStats (received from a client) processing and for watching that traffic is flows normally. Also in this PR is implemented service which put together stats from the Coturn and a client, and based on this stats makes decision about Peer's traffic flowing.

This PR is just part of the on_start/on_stop Control API callbacks implementations. It's doesn't adds new functional and needed only for the code review simplification.

Solution

Pick peers module and important for this module things from the #91.

Checklist

• Created PR:
  • In draft mode
  • Name contains WIP: prefix
  • Name contains issue reference
  • Has k:: labels applied
  • Has assignee
• Documentation is updated (if required)
• Tests are updated (if required)
• Changes conform code style
• CHANGELOG entry is added (if required)
• FCM (final commit message) is posted
  • and approved
• Review is completed and changes are approved
• Before merge:
  • Milestone is set
  • PR's name and description are correct and up-to-date
  • WIP: prefix is removed
  • All temporary labels are removed

[evdokimovs]

FCM

Add traffic watcher services for the 'on_start'/'on_stop' callbacks (#98, #27)

- add service which puts together client's stats and Coturn's stats
- add service for the client's RTCStats processing
- add [media] section to app's config with traffic watcher timeouts

Additionally:
- fix #[enum_dispatchable] macro to work fine with multi-arg functions