Instead of wrapping all non-movable members of TransportSocket in OwnPtr
to keep it movable, make TransportSocket itself non-movable and wrap it
in OwnPtr.
When the request is stopped, we clear its internal stream data. There is
a window where RequestServer may have sent an IPC message whose callback
will try to access that data in the time between the data being cleared
and RS receiving the stop signal. When this happens, just bail from IPC.
This has been a longstanding ergonomic issue with our IPC compiler. Non-
trivial types were previously passed by const&. So if we wanted to avoid
expensive copies, we would have to const_cast and move the data.
We now pass ownership of all transferred data to the client subclasses.
This allows us to remove const_cast from these methods, and allows us to
avoid some trivial expensive copies that we didn't bother to const_cast.
