Consider warning if a final variable is changed via readMarshallable

[rogersimmons]

readMarshallable allows final variables to be changed - eg when reading config on startup.
As Java allows aggressive optimisation of final variables this can result in a situation where a variable in an object has one value (changed from config), but accessors will return a different value (a default baked in at compilation).

Ideally a warning should be emitted when a final variable is changed, or the config refused altogether.

[RobAustin]

We should be more abrupt and throw an exception, the problem with WARNings is they get missed!

[alamar]

Let's do warning now, error in 2.24?

[alamar]

Turns out we have more than 10 tests which do exactly that - overwrite final fields.

What should we do - should we update them all?

@rogersimmons @RobAustin

[RobAustin]

@alamar this looks done, if this is not the case please reopen

[alamar]

Nope, no real work is done yet, reopening.