As server, put node nick in GATT Characteristic User Description, so
that clients have some information which endpoint is which in case we
have multiple. This is not in BLE MIDI standard, but cannot hurt.
As client, make use of that information, if present.
Implement BLE MIDI node, providing an input and output port. MIDI data
received is normalized and produced on output port. Data from input
port is converted to BLE MIDI and sent to the device.
Event timestamps come from device clock, whose offset to wall clock we
determine by correlating packet reception times with their timestamps.
The jitter in packet reception is likely much larger than device clock
error, so we average over that.
