Expand benchmarks for Java

[nmittler]

From grpc/grpc-java#1054:

Implement some basic protobuf benchmarks independent of GRPC
(scrape the web or extract Googles) as a framework for experimenting
with optimizations

The gRPC team has a few ideas for optimizing protobuf marshalling but we need to expand the benchmark suite before that work can begin.

cc/ @louiscryan @ejona86 @xfxyjwf

[nmittler]

FYI, I'd be happy to do the work here.

[xfxyjwf]

The benchmarks for proto Java are not opensourced. I suggest we track this bug internally, add new benchmarks to our internal code base and then get part of these benchmarks opensourced if needed.

[xfxyjwf]

Also, for significant protobuf changes like this (the planned performance optimizations) we should do it internally first to make sure it does not impact google3 users negatively.

[nmittler]

@xfxyjwf sgtm ... I'll close this for now and continue internally. Thanks.