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.

docs/use/ble.md

@@ -164,7 +164,8 @@ The default value is set into config_BT.h
 
 The gateway can read and write BLE characteristics from devices and provide the results in an MQTT message.  
 ::: tip
-These actions will be taken on the next BLE connection, which occurs after scanning and after the scan count is reached, [see above to set this.](#setting-the-number-of-scans-between-connection-attempts)
+These actions will be taken on the next BLE connection, which occurs after scanning and after the scan count is reached, [see above to set this.](#setting-the-number-of-scans-between-connection-attempts)  
+This can be overridden by providing an (optional) parameter `"immediate": true` within the command. This will cause the BLE scan to stop if currently in progress, allowing the command to be immediately processed. All other connection commands in queue will also be processed for the same device, commands for other devices will be deferred until the next normally scheduled connection.
 :::
 
 ### Example write command
@@ -175,7 +176,8 @@ mosquitto_pub -t home/OpenMQTTGateway/commands/MQTTtoBT/config -m '{
   "ble_write_char":"cba20002-224d-11e6-9fb8-0002a5d5c51b",
   "ble_write_value":"TEST",
   "value_type":"STRING",
-  "ttl":4 }'
+  "ttl":4,
+  "immediate":true }'
 ```
 Response:
 ```