Add support for transport unbind to proton-j

As discussed on the mailing list recently (see http://qpid.2158936.n2.nabble.com/Defining-the-behaviour-of-Proton-Engine-API-under-error-conditions-tp7590533.html), Proton applications sometimes want to unbind a transport, e.g. following an error, with the intention of binding to a new transport later on.

proton-j does not currently support unbinding.