Incoming CANCEL transaction are never removed from ua.transactions.nist

[ibc]

• Receive an INVITE.
• Ring.
• Caller sends CANCEL.
• We reply 200 Cancelling and 487 and receive ACK as usual.

After that, and forever and ever, the corresponding CANCEL non INVITE transaction remains forever on ua.transactions.nist.

This prevents UA.stop() to work because it relies on nist to be 0 before it disconnects the transport:

/*
   * If the remaining transactions are all INVITE transactions, there is no need to
   * wait anymore because every session has already been closed by this method.
   * - locally originated sessions where terminated (CANCEL or BYE)
   * - remotely originated sessions where rejected (4XX) or terminated (BYE)
   * Remaining INVITE transactions belong tho sessions that where answered. This are in
   * 'accepted' state due to timers 'L' and 'M' defined in [RFC 6026]
   */
  if (this.nistTransactionsCount === 0 && this.nictTransactionsCount === 0) {
    this.transport.disconnect();
  } else {
    this.on('transactionDestroyed', transactionsListener);
  }

[ibc]

This is because UA.destroyTransaction() is never called for the incoming CANCEL.

[ibc]

The problem is that IncomingRequest.prototype.reply_sl() is used to reply the 200 to the CANCEL, and reply_sl does not notify the server transaction so it remains in nist forever.