WIP Refactor proto

[xiang90]

switch to use gogoprotobuf generated codes.
Gogoprobuf will generate helper codes that do not use reflection.
Performance improvement about 50% for the switching.
Another 100%+ improvement for refactoring.

Before using gogoprotobuf
BenchmarkAppendEntriesRequestEncoding 1000 1806812 ns/op 74.17 MB/s
BenchmarkAppendEntriesRequestDecoding 1000 2217673 ns/op 60.43 MB/s
BenchmarkAppendEntriesResponseEncoding 1000000 1424 ns/op 5.62 MB/s
BenchmarkAppendEntriesResponseDecoding 1000000 3434 ns/op 2.33 MB/s

After using gogoprotobuf
BenchmarkAppendEntriesRequestEncoding 2000 1084766 ns/op 123.54 MB/s
BenchmarkAppendEntriesRequestDecoding 1000 1715724 ns/op 78.11 MB/s
BenchmarkAppendEntriesResponseEncoding 5000000 636 ns/op 12.57 MB/s
BenchmarkAppendEntriesResponseDecoding 1000000 2469 ns/op 3.24 MB/s

After refactoring (stop copying around)
BenchmarkAppendEntriesRequestEncoding 5000 367500 ns/op 364.67 MB/s
BenchmarkAppendEntriesRequestDecoding 2000 1448770 ns/op 92.50 MB/s
BenchmarkAppendEntriesResponseEncoding 5000000 642 ns/op 12.46 MB/s
BenchmarkAppendEntriesResponseDecoding 1000000 2225 ns/op 3.59 MB/s

There are still some improving space. I am working on this.

[xiang90]

@benbjohnson There are still some cleaning up stuff to work on. But you can review this pr to get a rough idea what I am doing.

[benbjohnson]

@xiangli-cmu That looks good to me. Can we remove the Proto prefix from all the type names? It seems redundant since they're already in the protobuf package.

[xiang90]

@benbjohnson Will do.

[philips]

Does this change the protocol or log format?

[xiang90]

@philips No. I did not change any format.

[xiang90]

@benbjohnson Can we merge this? I have some more cleaning up work to do. But it is better to do in another pull request I think. Thanks.

[benbjohnson]

👍