bluez5-device: Rewrite of thread function, reduce send buffer size for a2dp sink

bluez5-device: Rewrite of thread function, reduce send buffer size for a2dp sink

The rewrite of the thread function does not change functionality much,
most of it is only cleanup, minor bug fixing  and documentation work.

This patch also changes the send buffer size for a2dp sink to avoid lags
after temporary connection drops, following the proof-of-concept patch
posted by Dmitry Kalyanov.

Link: https://bugs.freedesktop.org/show_bug.cgi?id=58746
Additionally the patch changes the fixed latency for HSP playback from 125
to 25 ms. Tests showed that this produces better audio sync, which is
expected as HSP should have smaller latency than A2DP.