Fix RpcClient reference leak in Jason (#27)

[evdokimovs]

Part of #27

Synopsis

After RpcClientwas refactored to async_trait usage in #135, RpcClient::on_normal_close starts require one more reference to the RpcClient. Because of it, RpcClient remains alive after Room close.

Solution

1. Return LocalBoxFuture from the RpcClient::on_normal_close instead of async fn usage

2. Add unit tests for this bug

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]

Патч с новыми юнит тестами для проверки того, что они падают на мастере. Применять патч нужно на master ветке.

[evdokimovs]

FCM

Fix RpcClient reference leak in Jason (#140, #27)