RTC-504 RecordingEndpoint sends finished action after all input tracks unlink

[Rados13]

This PR consists of two parts:

• change the behavior of RecordingEndpoint so it will remove itself from the engine after all input tracks are removed.
• fix a bug in the engine regarding handling subscribe messages from an endpoint that does not exist. When the engine receives a subscribe message from an existing endpoint, it will throw an exception because we are trying to refer to non-existing data. It is fixed by checking if the endpoint that tries to subscribe on track exists in the engine, and if not, a proper message is returned to the endpoint.

[codecov[bot]]

Codecov Report

Merging #381 (91b19c5) into master (3dde5e0) will increase coverage by 0.50%. The diff coverage is 40.90%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #381 +/- ## ========================================== + Coverage 53.59% 54.09% +0.50% ========================================== Files 62 61 -1 Lines 2685 2671 -14 ========================================== + Hits 1439 1445 +6 + Misses 1246 1226 -20 ``` | [Files](https://app.codecov.io/gh/jellyfish-dev/membrane_rtc_engine/pull/381?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jellyfish-dev) | Coverage Δ | | |---|---|---| | [hls/lib/hls\_endpoint.ex](https://app.codecov.io/gh/jellyfish-dev/membrane_rtc_engine/pull/381?src=pr&el=tree&filepath=hls%2Flib%2Fhls_endpoint.ex&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jellyfish-dev#diff-aGxzL2xpYi9obHNfZW5kcG9pbnQuZXg=) | `50.90% <ø> (ø)` | | | [sip/lib/sip\_endpoint.ex](https://app.codecov.io/gh/jellyfish-dev/membrane_rtc_engine/pull/381?src=pr&el=tree&filepath=sip%2Flib%2Fsip_endpoint.ex&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jellyfish-dev#diff-c2lwL2xpYi9zaXBfZW5kcG9pbnQuZXg=) | `0.00% <ø> (ø)` | | | [webrtc/lib/webrtc\_endpoint.ex](https://app.codecov.io/gh/jellyfish-dev/membrane_rtc_engine/pull/381?src=pr&el=tree&filepath=webrtc%2Flib%2Fwebrtc_endpoint.ex&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jellyfish-dev#diff-d2VicnRjL2xpYi93ZWJydGNfZW5kcG9pbnQuZXg=) | `0.00% <0.00%> (ø)` | | | [recording/lib/recording\_endpoint.ex](https://app.codecov.io/gh/jellyfish-dev/membrane_rtc_engine/pull/381?src=pr&el=tree&filepath=recording%2Flib%2Frecording_endpoint.ex&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jellyfish-dev#diff-cmVjb3JkaW5nL2xpYi9yZWNvcmRpbmdfZW5kcG9pbnQuZXg=) | `0.00% <0.00%> (ø)` | | | [engine/lib/membrane\_rtc\_engine/engine.ex](https://app.codecov.io/gh/jellyfish-dev/membrane_rtc_engine/pull/381?src=pr&el=tree&filepath=engine%2Flib%2Fmembrane_rtc_engine%2Fengine.ex&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jellyfish-dev#diff-ZW5naW5lL2xpYi9tZW1icmFuZV9ydGNfZW5naW5lL2VuZ2luZS5leA==) | `82.01% <50.00%> (-1.21%)` | :arrow_down: | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/jellyfish-dev/membrane_rtc_engine/pull/381/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jellyfish-dev) ------ [Continue to review full report in Codecov by Sentry](https://app.codecov.io/gh/jellyfish-dev/membrane_rtc_engine/pull/381?dropdown=coverage&src=pr&el=continue&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jellyfish-dev). > **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jellyfish-dev) > `Δ = absolute (impact)`, `ø = not affected`, `? = missing data` > Powered by [Codecov](https://app.codecov.io/gh/jellyfish-dev/membrane_rtc_engine/pull/381?dropdown=coverage&src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jellyfish-dev). Last update [3dde5e0...91b19c5](https://app.codecov.io/gh/jellyfish-dev/membrane_rtc_engine/pull/381?dropdown=coverage&src=pr&el=lastupdated&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jellyfish-dev). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jellyfish-dev).

[mickel8]

Please add some PR description :pray: