Add immediate parameter to BLE connection commands. (#1137)

* Add immediate parameter to BLE connection commands.

When sending a command to read/write a characteristic on a BLE server this adds a new parameter to cause the action to be
performed immediately. This will stop the scan in progress to perform the actions queued.

Example use:
```
mosquitto_pub -t home/OpenMQTTGateway/commands/MQTTtoBT/config -m '{
  "ble_write_address":"AA:BB:CC:DD:EE:FF",
  "ble_write_service":"cba20d00-224d-11e6-9fb8-0002a5d5c51b",
  "ble_write_char":"cba20002-224d-11e6-9fb8-0002a5d5c51b",
  "ble_write_value":"TEST",
  "value_type":"STRING",
  "ttl":4,
  "immediate":true }'
```

Stop BLE processing when immeditate command sent.

* Use a shorter connection timeout.

* Update docs.

* Remove MQTT spam for connection retrys

* Prevent crashing when the normal connection process occurs at the same time as an immediate action.

* Use a semaphore to protect BLE device vector.

main/ZgatewayBLEConnect.h

@@ -12,7 +12,10 @@ class zBLEConnect {
 public:
   NimBLEClient* m_pClient;
   TaskHandle_t m_taskHandle = nullptr;
-  zBLEConnect(NimBLEAddress& addr) { m_pClient = NimBLEDevice::createClient(addr); }
+  zBLEConnect(NimBLEAddress& addr) {
+    m_pClient = NimBLEDevice::createClient(addr);
+    m_pClient->setConnectTimeout(5);
+  }
   virtual ~zBLEConnect() { NimBLEDevice::deleteClient(m_pClient); }
   virtual bool writeData(BLEAction* action);
   virtual bool readData(BLEAction* action);