Remove Coturn credentials on IceUser drop in Medea (#27)

[evdokimovs]

Part of #27

Synopsis

Currently we doesn't remove IceUser credentials from Coturn on Peer remove. But for security reasons we need to do it.

Solution

Remove Coturn credentials and remove all session on IceUser drop

Checklist

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

[evdokimovs]

FCM

Remove Coturn credentials on IceUser drop in Medea (#176, #27)

Additionally:
- fix RpcConnections not being removed in some cases