Fix/ble stop scanning

[avtolstoy]

Problem

1. BLE.stopScanning() on Gen 4 is broken since 5.8.2 from inside ble scan result callback.
2. In general behavior of Gen 3/4 scan() and stopScanning() is currently slightly different. Also different with indefinite/timed scans.
3. No-fixture tests are missing for this.

Solution

1. Fix this by making sure that only the scan caller semaphore is unblocked if hal_ble_gap_stop_scanning() is called from inside the ble event thread (the one ble results are being notified to the application from)
2. Streamline the behavior to be:
   2.1. BLE.scan() is blocking no matter the timeout, but once scanning is started - BLE lock is not held, allowing other BLE operations to be performed.
   2.2. BLE.stopScanning() works correctly when called both from inside the scan result callback/other BLE callbacks and from some other thread (be that application/system/timer/custom etc).
   2.3. If called from a normal thread BLE.stopScanning() is a blocking operation and scanning MUST be stopped once its execution completes.
3. Add a set of tests to validate scan/stopScanning.

Steps to Test

• wiring/no_fixture_ble
• wiring/ble_central_peripheral
• wiring/ble_scanner_broadcaster

For cross-platform testing with test runner: particle-iot/device-os-test-runner#28

Example App

void setup() {
  /* A minimal example app is super helpful 
   * for testing new features and fixes. 
   * A link to a Docs PR is even better!
   */
}

void loop() {

}

References

Links to the Community, Docs, Other Issues, etc..

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)

[XuGuohui]

1. no_fixture_ble passed
2. ble_central_peripheral passed