context: add option for soft/hard clock rate changes

Hard rate changes (the default now) makes the graph suspend so that the
driver reopens the device with the new sample rate if possible. This
causes a glitch.

Soft rate changes works as before and just changes the clock rate of
the position area, which makes everything resample things to the new
rate without a glitch.

Sample rate changes can currently still only be triggered by
configuring the settings metadata clock.force-rate