[policy] Allow dynamically activated streams.

In configuration you may define "sink" for "stream", and when that
routing target is set for sink, group becomes active and vice versa.
Dynamically changing streams need to be defined before similar streams
in the policy configuration file.
[sink-input-ext] Apply volume factor in sink_input_put().

Previously volume factor was applied after sink_input_new(), and at that
time it is not guaranteed that sink-input has its sink or channel_map
etc defined. Apply volume factor later, after sink_input_put(), when
these details are known.
Merge pull request #9 from jusa/dev

Volume factor fix and dynamic streams.