1. Introduce ble.Device interface, so we can mock it,
   and have other platform independent implmentation such
   as grpc-based one.

2. Promote GATT stuff from ble/examples/lib/gatt to ble/

3. Make API more synchronous:

  a. Remove StopAdvertising, StopScanning, ..
  b. Add context.Context in Scanning, AdvertiseXX API.

  So,  user can easily have a blocking advertising/scanning with
  optionally set timeout.

4. Update current examples accordingly, and move them to
   lib/examples/basic/