Add service for Coturn stats

[evdokimovs]

Part of #27

Required for #91

Synopsis

We need to process Coturn's stats for the on_start/on_stop Control API callbacks (#91). This PR adds service which will fetch and deserialize all Coturn stats events.

Also we need to change algorithm of a new Coturn user creating. Now Coturn users will be created individually for all PeerConnections. This is needed for convenient processing of a PeerConnection's Coturn stats.

Solution

• 1. Change algorithm of a new Coturn user creating
• 2. Add CoturnStats service
  • 2.1. Coturn events deserialization
  • 2.2. Reconnection to a Coturn stats Redis server
  • 3.3. Detecting PeerConnection's traffic starting and allocations deleting

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 service for Coturn stats (#94, #27)

- register Coturn credentials for each peer
- add service for Coturn stats processing