make belle-sip channel to enter retry state when a REGISTER transaction gets a 500 Server Internal Error

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.

This reverts commit 2722edef.

modifications of http chunked mode (receiving): each segment of data received is immediately pushed to the body, without waiting for the end of the chunk.
This makes it more tolerant to servers sending big chunks of data.

progress in http implementation

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

(to be tested).

transaction weak refs the dialog, so that is not necessary to unref when the transaction terminates.

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)

enhance nat helper mechanism so that even unchallenged requests can be immediately resubmitted by the refresher if the contact could not be properly set the first time.

 - Use size_t instead of unsigned int for offset.
 - Add function to enable marshal check at runtime.
 - Enable marshal check in unit tests.

- connection fallback mechanism in progress.

allow transaction to kill dead channels in case of timeout