Fix issues on poor network and poor Modbus controllers

[AkLim94]

Dear community,

I experienced some issues when using this excellent component.
The issues appended especially on poor network quality and probably poor controllers (with strange behaviors).
I made some changes in the code to fix these issues and would like to share with you, but unfortunately how to contribute....
If anyone know how to do, it's welcome

The changes made:

• Check MBAP header with the length received
• Added more events (to be able to display Modbus frames)
• Clear input buffer before to send command (to be sure what we receive is for the command we've just sent and not for the previous command, yes some controllers response several times the same)

Regards