LibProtocol::Client::start_download() now gives you a Download object
with convenient hooks (on_finish & on_progress).
Also, the IPC handshake is snuck into the Client constructor, so you
don't need to perform it after instantiating a Client.
This makes using LibProtocol much more pleasant. :^)
The DownloadFinished message from the server now includes a buffer ID
that can be mapped into the client program.
To avoid prematurely destroying the buffer, the server will hang on to
it until the client lets it know that they're all good. That's what the
ProtocolServer::DisownSharedBuffer message is about.
In the future it would be nice if the kernel had a mechanism to allow
passing ownership of a shared buffer along with an IPC message somehow.
