fixed java codegen bug documented in #4563 (#4565)

* grpc bindings generator for Java and a few minor supporting changes in improvements

* restored formatting before my previous changes for ease of review

* Fixed grpc java code generation bug resulting in duplicate extractor declarations in case the same is used in more than a single RPC method

* fixed previous merge issue

* removed extra space

* restored extra space

* restored extra space

* fixed java codegen bug documented in https://github.com/google/flatbuffers/issues/4563