Towards a new disk-reader ringbuffer

Towards a new disk-reader ringbuffer

This is mainly a NO-OP, introducing a new PlaybackBuffer type
and preparing for its use.

At this point in time, the buffer is just a power-of-two sized
ringbuffer and the disk-reader's read-logic is still unchanged.

Eventually the read and write sample position that are currently
private to the disk-reader can be migrated to be owned by the buffer.
Also Diskreader::read() positions can be matched to read-position ..
+/- buffer reservation and de-click can read w/o committing the read.