[FR] Add ModBus handler for all Modbus plugins

[TD-er]

Use emelianov / modbus-esp8266 library:
https://github.com/emelianov/modbus-esp8266

This library allows for queueing requests with callback functions.
For callback example, see: emelianov/modbus-esp8266#165

ToDo:

• Add ModbusHandler which makes several instances of this modbus object based on used (serial) port
• Make instances of modbus object use something like smart pointer to make sure it gets destructed when the last user (task) exits
• Add separate task for ModbusHandler on RTOS to make it read completely async.
• Add modbus sensor device type DEVICE_TYPE_MODBUS_RTU
• Add queue mechanism with callback
• Add extra reg reads for use in loading web form which may need to be pre-loaded or periodically read and cached.
• Mark which regs should be read only once (e.g. serial nr)
• Check boxes fixed in this issue: [Cleanup] Get rid of 'blocking' code like delay() #4103