fix(visitors): Make sure we disconnect vnodes before leaving rooms.

[damencho]

Use case is a single main participant and a visitor node. If jicofo leaves visitor room before disconnecting, the client will reload seeing jicofo leaving. If we disconnect first, we destroy the room with an appropriate message shown to visitors.

[codecov]

Codecov Report

Attention: Patch coverage is 62.50000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 27.56%. Comparing base (ac6cb05) to head (b6ee464).
Report is 22 commits behind head on master.

Files with missing lines	Patch %	Lines
...tsi/jicofo/conference/JitsiMeetConferenceImpl.java	76.92%	3 Missing ⚠️
...in/kotlin/org/jitsi/jicofo/xmpp/VisitorsManager.kt	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1170      +/-   ##
============================================
- Coverage     29.24%   27.56%   -1.69%     
+ Complexity      485      476       -9     
============================================
  Files           130      129       -1     
  Lines          7915     7967      +52     
  Branches       1087     1105      +18     
============================================
- Hits           2315     2196     -119     
- Misses         5326     5488     +162     
- Partials        274      283       +9     

Files with missing lines	Coverage Δ
...tsi/jicofo/conference/JitsiMeetConferenceImpl.java	41.41% <76.92%> (-3.00%)	⬇️
...in/kotlin/org/jitsi/jicofo/xmpp/VisitorsManager.kt	0.00% <0.00%> (ø)

... and 4 files with indirect coverage changes

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