Try to allow building java without having to build codegen

[ejona86]

It may be possible to use the last released codegen binary.

[zhangkun83]

Some thoughts:

• Run a continuous build service (e.g., Jenkins) to build codegen and push it to snapshot repo for every commit. We will need three machines (Linux, Mac and Windows) for this.
• Disable grpc-compiler by default when building the root project. If grpc-compiler is disabled, other subproject will use the codegen with the same version number as the current code. So if the code is SNAPSHOT, codegen will be from the snapshot repo. This should relieve a lot of pain for most contributors who don't touch the codegen.
• Contributors that do change the codegen must use an option to enable building compiler locally. Alternatively they could install their changed codegen locally.

[ejona86]

I thought of another option that I think could work well and isn't hacky: we check in the generated code. We make code generation not part of "compile" and only run it when we are modifying the code generator.

[zhangkun83]

This is actually a good idea. Where do you think we should put the generated code? I am thinking src/generated/java/

[ejona86]

SGTM.