Don't wrap the entire send/response cycle in a counting semaphore.

While the counting semaphores may be redundant anyway (needs to be
confirmed, but bus_send_request is already blocking until the send is
complete, with extra backpressure), additinoally holding on to the count
until the response arrives and the unpacked message callback is called
limits the total number of responses that can be pending, which is not
the intention. The message bus itself should do that, but allow for
significantly more pipelining than the counting semaphor with a limit
of 10.