Support for CANCELing request before it's being proxied

in case of asynchronous processing of Sip Request it's possible for
CANCEL to arrive before SipProxy is started. In this case SipProxy has
to handle CANCEL itself (e.g. send REQUEST_TERMINATED).
fix NPE

In some cases when transaction is cancelled hops returned will be null
causing exception logged
If there is no linked session no need to create derived session

there is possibility that linked session is invalidated which causes
inability to create derived session for it.
Better message on what did happen

In case of MessageTooLong it's helpful to see that state of ByteBuffer
being overflown.