Expose max coil/register constants, enforce in API

[jadamcrain]

Two options for enforcing...

A) throw exception in request constructors, i.e. fail fast
B) check after posting to strand and make error callback

thoughts?

I kind of lean towards A) b/c then you always know these are properly constructed.

[larrylackey]

Fail fast sound good, but not sure how this relates to automatically combining Modbus adjacent writes or reads with small gaps

[jadamcrain]

We decided to have an error callback so as not to mix error handling paradigms. Nothing else in the library throws an exception in response to a method call.