fix: Fix a leak of WebSocketClients.

[bgrozev]

Every time we establish an active WS to another relay we create a new
WebSocketClient instance. This comes with its own QueuedThreadPool with 8 threads
and if we lose the reference without calling stop() it leaks.

When the existing web socket closing arrives before the signaling that
expires the relay, we try do WebSocketClient.stop() and then
doConnect(). The call to stop() can take a while, and if in the meantime
the relay is expired in signaling we end up running doConnect() on
an expired relay. In this case we create a new WebSocketClient, which
is never stopped.

This fix uses a single WebSocketClient instance shared between all
relays, and does not attempt to re-connect if the reason for the
disconnect is RELAY_CLOSED. It re-uses the "webSocket: ColibriWebSocket"
field for sockets created both actively and passively.

[codecov]

Codecov Report

Merging #2051 (a102e43) into master (8983b11) will increase coverage by 0.13%.
The diff coverage is 10.00%.

@@             Coverage Diff              @@
##             master    #2051      +/-   ##
============================================
+ Coverage     42.41%   42.55%   +0.13%     
- Complexity     1673     1682       +9     
============================================
  Files           325      325              
  Lines         17155    17142      -13     
  Branches       2255     2253       -2     
============================================
+ Hits           7277     7294      +17     
+ Misses         9168     9132      -36     
- Partials        710      716       +6     

Files Changed	Coverage
...g/jitsi/videobridge/relay/RelayMessageTransport.kt	10.00%

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