[Java] Do not generate setter method for readonly properties

[xhh]

Updated Java clients: default, feign, jersey2, okhttp-gson.

[hiveship]

I will also add it to CXF Codegen :)
I also think that if property is 'readOnly: true' we can declare it as 'final'. Are you ok with me @xhh ?

[ePaul]

Note that readOnly for a Swagger property just means that the client shouldn't send it to the server, but the server can send it back.
So please make sure it doesn't remove the setter when generating server models.

[hiveship]

Ok sorry @ePaul . I won't remove the setter in the CXF Codegen since it only generates a server stub :)

[wing328]

FYI. The change applies to retrofit and retrofit2 as well.

cc @cbornet

[wing328]

Upgrade note from 2.1.5 to 2.1.6

Starting from 2.1.6, there will be no setter for readOnly attribute in Java models.