* Connection implements async context manager to disconnect when
context is left
* The Connection only calls disconnect if the context manager exits
without an exception
* Peer implements async context manager to discover when entering the
context
* Device.connect_as_gatt implements an async context manager to nest the
connection and peer context managers
* Added HCI_StatusError that can be raised when a HCI Command Status
event is received that doesn't show "PENDING" as status
* Added Connection.sustain to wait for a timeout or disconnect
* Peer.sustain also maps to Connectin.sustain
* Updated battery_client.py to use .connect_as_gatt and .sustain
* Updated heart_rate_client.py to use .connect_as_gatt and .sustain
