context: implement dynamic rate

Add a default.clock.allowed-rates property that lists the possible
rates for the graph.

Add node.rate and node.lock-rate properties to suggest a graph rate
and lock the rate.

Collect the largest clock rate from the nodes and if it is allowed,
try to configure it as the graph rate. Only switch rates when the
driver is IDLE or suspended or when we force a rate.

No alternative samplerates are specified because we first need to
work around a common driver bug (cards with 1 cristal need the same
rate for capture as playback) for this to work.