when pending transactions are queued in the dialog, newly created transaction must be automatically queued as well, in order to preserve request order.
Previous commit broke the normal way of sending a request through a transaction by not honoring the "outbound proxy" argument.

This reverts commit 8b1a7977.

improvements of the dialog's request queue in order to allow urgent request to be submitted before processing of the queue.

(to be tested).

implement transaction queue in dialog, to avoid rejecting new requests while there is pending transaction within this dialog.
remove circular references between dialog and transaction
optimize dialog matching for client transaction (avoid lookup in dialog list)

moment.