Start isolating control block accesses in a proxy

The proxy object will eventually be the only code that understands the
details of the control block.  This should make it easier to change the
control block in the future.

Initial set of control block fields that are isolated:
 - sample rate
 - send level
 - volume

Prepare for streaming/static separation by adding a union to the control
block for the new fields.

Fix bug in handling of max sample rate on a track.  It was only checking
at re-configuration, not at each mix.

Simplify OutputTrack::obtainBuffer.

Change-Id: I2249f9d04f73a911a922ad1d7f6197292c74cd92