bluetooth: allow increasing output bitrate

Bluetooth thread may ask encoder to reduce bitrate if writing is not keeping up
with inputs or writing to bluetooth socket takes too much time.

Assuming conditions leading to reduced bitrate are intermittent, allow periodic
attempts to increase encoder bitrate, by default at most twice per second.

Part-of: <pulseaudio/pulseaudio!481>
3 jobs for pr/add-increase-bitrate-method in 4 minutes and 58 seconds (queued for 1 second)