Deinit UARTService CharacteristicBuffers on disconnect

[dhalbert]

• Fixes Memory leak after repeated BLE reconnection and UARTService activation #192.

CharacteristicBuffer must be deinit'd after use, to prevent a storage leak. Do this for UARTService, and provide a mechanism to invoke deinit() on disconnect().

test to detect storage leak:

import os, gc
import time
import board
from adafruit_ble import BLERadio
from adafruit_ble.services.nordic import UARTService
from adafruit_ble.advertising.standard import ProvideServicesAdvertisement

ble = BLERadio()
uart = UARTService()

while True:
    print("Scanning ...")
    for adv in ble.start_scan(ProvideServicesAdvertisement, timeout=1):
        if UARTService not in adv.services:
            continue
        count = 1
        while True:
            try:
                conn = ble.connect(adv)
                svc = conn[UARTService]
                print(f"{count} Free memory: {gc.mem_free()}, Allocated memory: {gc.mem_alloc()}")
                conn.disconnect()
                conn = None
                svc = None
                gc.collect()
            except Exception as e:
                print("Error: ", e)
            count += 1

[dhalbert]

Could someone review this? Thanks.

[tekktrik]

Looks good to me!