Gen4 BLE fixes

[XuGuohui]

Problem

1. connectionsMutex_ may block the application thread for prolonged amount of time. For example:
   • onBLEPairingEvent()hold the connectionsMutex_
   • waiting for some cloud data in the BLE pairing event handler
   • received data from cloud and the subscription handler is scheduled from application thread
   • application thread calls waitFor(][(return BLE.isPaired)), which is blocked by connectionsMutex_
   • cloud data cannot be handled
2. assertTrue(results[i].scanResponse().length() > 0); may fail in ble_central_peripheral/ble_central test

Solution

1. Do not acquire connectionsMutex_ in notifyLinkEvent() and acquire it where necessary.
2. To be consistent with Gen3, BLE HAL doesn't notify the pending scanned adv data

Example App

user/tests/wiring/ble_central_peripheral

References

N/A

---

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)