ble: fix some issues in connect and disconnect

[gen2thomas]

Solved issues and/or description of the change

• see BLE: nil pointer dereference on disconnect possible #1157 and BLE: some missing code on Connect() and Disconnect() #1160
• introduced an option to drop all characteristics on each disconnect

Manual test

• OS and Version (Win/Mac/Linux): Linux
• Adaptor(s) and/or driver(s): bleclient on a pine64 rock64

Checklist

• The PR's target branch is 'hybridgroup:dev'
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes (e.g. by run make test_race)
• No linter errors exist locally (e.g. by run make fmt_check)
• I have performed a self-review of my own code