Bitrate adjustment based on segment filesize.

When a .ts segment is written, inspect the filesize and
scale video bitrate to match the expected filesize at target bitrate.
This algorithm is mindful of the available bandwidth.

There is now a tick-tock bitrate adjustment:
On .ts segment written, adjust bitrate to meet expected filesize, but
not exceed last observed bandwidth capability.
On .ts segment upload, update observed bandwidth capability
and lower bitrate if necessary to maintain live streaming.