No Delay between polls #179

Some modbus devices does not respond well when multiple requests are done with no delay between them.
NModbus4 doesn't have any parameters to set Delay between polls.
It will help to query all commands one by one after setting Delay Between Polls time by user even if user write or read multiple queries at same time.
Option for modbus data sources could be added to configure the Delay Between Polls (from zero, meaning no delay, to something like 500 or 1000 ms).