Thingsboard-gateway

* Fix for deadlock on uploading telemetry.

Minor release with the following improvements.

**Improvements:**
* [Connectors] Improvements to RPC requests and attribute update requests processing;
* [TB gateway service] Improvements in configuration processing;
* [Request Connector] Improvements for data processing from external API;
* Code refactor.

**Bug fixes:**

* [Modbus Connector] Fix for 262;
* [Modbus Connector] Fix for 261;
* [Custom serial connector] Fix for device processing;

**Fixes and improvements**:
- Minor fixes for Modbus Connector
- Partial refactoring of the gateway service.

Fixes:
- Fix for 255 Resolved issue with expressions in the converter.