Drop artificial limit on 2 channels.

- Not yet tested with real multiple channels, but doesn't die in fires.
- Bumped version to 2.1

Tests & Fixes.

- Handle silence detection vs stream time differently.
- Restore "max_delay" detection.
- Fix ms/s unit in output.

Replace a deprecated alias "async" with ensure_future.

- ref: https://docs.python.org/3.6/library/asyncio-task.html
- Found by kolorafa - many thanks!
- Makes wavesync compatible with python 3.7
- Should not break backward compatibility.

Bump version to 1.1.0