Limit the number of concurrent connections on MODBUS TCP

[ziotibia81]

This is another scenario related to #1664

Some MODBUS devices support very limited concurrent TCP connections. For example, I'm using a device that supports only two connections. Sometimes a connection to a device remains open without exchanging data. When the open connections reach the device connection limits, the gateway is no longer able to read data until a gateway or device restarts.

[imbeacon]

Hi @ziotibia81,

Keeping connection is by design, the gateway keeps connection to device to have ability to control it’s activity on ThingsBoard and be able to process incoming shared attributes and RPC correctly, so it is not possible to connect each time, also it will cause a lot of connection/disconnection messages between the gateway and device.