Fix a race condition in startNotification with the "valuechanged" event notifications

[nmasse-itix]

This Pull Request is a proposal to fix #72 by removing a race condition between startNotification and the "valuechanged" event notifications.

The unit test wraps the StartNotify call with an early event emission.
An assertion is positioned to ensure that this early event is not lost.

With the original code, the new unit test fails with:

$ npm run test:jest

> node-ble@1.12.0 test:jest
> jest --testPathIgnorePatterns=test-e2e/

 PASS  test/GattService.spec.js
 PASS  test/buildTypedValue.spec.js
 PASS  test/GattServer.spec.js
 PASS  test/parseDict.spec.js
 PASS  test/Device.spec.js
 PASS  test/Bluetooth.spec.js
 PASS  test/BusHelper.spec.js
 FAIL  test/GattCharacteristic.spec.js
  ● race condition between event:valuechanged / startNotification

    expect(received).toHaveLength(expected)

    Expected length: 2
    Received length: 1
    Received array:  [[{"data": [98, 97, 114], "type": "Buffer"}]]

      129 |
      130 |   // Check the mocked callback function has been called twice
    > 131 |   expect(cb.mock.calls).toHaveLength(2);
          |                         ^
      132 |   
      133 |   // Cleanup
      134 |   characteristic.off('valuechanged', cb)

      at Object.toHaveLength (test/GattCharacteristic.spec.js:131:25)

 PASS  test/Adapter.spec.js

Test Suites: 1 failed, 8 passed, 9 total
Tests:       1 failed, 37 passed, 38 total
Snapshots:   0 total
Time:        1.84 s, estimated 2 s
Ran all test suites.

With the proposed code, all tests are passing.

[coveralls]

coverage: 96.247%. remained the same
when pulling 381cd1b on Demo-AI-Edge-Crazy-Train:issue-72
into f936585 on chrvadala:main.