[QUIC] Improved logic of listener AcceptConnection

[ManickaP]

Addresses several problems with accepting incoming connections:

• pending connections now don't get queued until the handshake is finished (whether successfully or not)
  • ListenBacklog limit is enforced manually via _pendingConnectionsCapacity
  • in case of listener disposal, the active handshakes get stopped via _disposeCts
  • PendingConnection class is now irrelevant thus removed, _acceptQueue either contains QuicConnection or an Exception
• unexpected exception from handshake timeout is now wrapped in QuicException
  • this is straightforward solution without any regards to planned exception revisions where we should revisit this particular change ([QUIC] Revisit exceptions in Quic #82262)

Fixes #77310
Fixes #80172
Contributes to #78096

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Issue Details

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Addresses several problems with accepting incoming connections:

• pending connections now don't get queued until the handshake is finished (whether successfully or not)
  • ListenBacklog limit is enforced manually via _pendingConnectionsCapacity
  • in case of listener disposal, the active handshakes get stopped via _disposeCts
  • PendingConnection class is now irrelevant thus removed, _acceptQueue either contains QuicConnection or an Exception
• unexpected exception from handshake timeout is now wrapped in QuicException
  • this is straightforward solution without any regards to planned exception revisions where we should revisit this particular change (relevant issues: [API Proposal] QuicError.AcceptConnectionFailed #75184, [QUIC] Surface transport error code in QuicException #72666, QuicConnection.ConnectAsync raises SocketException when host not found #78751)

Fixes #77310
Fixes #78096
Fixes #80172

Author:	ManickaP
Assignees:	-
Labels:	area-System.Net.Quic
Milestone:	-

[wfurt]

LGTM.
I left few comments but they can be done independently. (certainly the documentation)

[wfurt]

and test failures seems relevant.

[ManickaP]

/azp run runtime-libraries-coreclr outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[ManickaP]

Failures are unrelated.