IP<->UART Modbus gateway (Modbus TCP/UDP over RS485).

Bridges Modbus TCP/UDP clients (port 502) to a Modbus RTU device on a UART (RS485). Incoming TCP/UDP requests are queued, sent serially with inter-frame delays, and the RTU responses are matched back to the originating client by MBAP transaction/unit ID. Based on the arduino-modbus-rtu-tcp-gateway design.