Use the sess.latency.msec also for the sender and use it to control the
NODE_LATENCY. Make it a float to be in line with the other time values.
Set is to a default of ptime, which was what it used to be.
This makes it possible to set the ptime to a smaller value than the
sess.latency.msec so that we send out multiple packets per quantum.
This will result in some bursty output for now but with a timer that can
be improved later.
Update the docs a little, mention the new rtp.ptime and rtp.frametime.
This method can be used to access the param_changed method of the
underlying pw_stream.
Also adds new public functions rtp_stream_set_param and
rtp_stream_update_params which plum things through to pw_stream_set_param
and pw_stream_update_params respectively.
