[RTC-226] Remove peers

[LVala]

This PR removes the concept of peers from membrane_rtc_engine in favour of endpoints keeping the metadata. Introduces breaking changes in MediaEvent API.

Requires this PR to be merged to pass integration tests.

[codecov[bot]]

Codecov Report

Merging #273 (4a00a9a) into master (9116fa9) will decrease coverage by 0.30%. The diff coverage is 51.66%.

@@            Coverage Diff             @@
##           master     #273      +/-   ##
==========================================
- Coverage   62.63%   62.34%   -0.30%     
==========================================
  Files          43       42       -1     
  Lines        2093     2090       -3     
==========================================
- Hits         1311     1303       -8     
- Misses        782      787       +5     

Impacted Files	Coverage Δ
...rane_rtc_engine/display_manager/display_manager.ex	0.00% <ø> (ø)
lib/membrane_rtc_engine/endpoints/hls_endpoint.ex	76.82% <ø> (ø)
lib/membrane_rtc_engine/endpoints/rtsp_endpoint.ex	86.44% <ø> (+1.69%)	:arrow_up:
...b/membrane_rtc_engine/endpoints/webrtc_endpoint.ex	0.00% <0.00%> (ø)
lib/membrane_rtc_engine/metrics.ex	0.00% <ø> (ø)
lib/membrane_rtc_engine/track.ex	88.88% <ø> (ø)
lib/membrane_rtc_engine/utils.ex	12.12% <ø> (ø)
...embrane_rtc_engine/endpoints/webrtc/media_event.ex	22.22% <5.55%> (-2.03%)	:arrow_down:
lib/membrane_rtc_engine/engine.ex	77.21% <92.85%> (+0.60%)	:arrow_up:
lib/membrane_rtc_engine/endpoint.ex	50.00% <100.00%> (-25.00%)	:arrow_down:
... and 1 more

... and 3 files with indirect coverage changes

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[LVala]

Changed media event leave to disconnect in the latest commit, which requires this changes in membrane-webrtc-js to be merged.