Replace rust-protobuf with prost

[ice1000]

This PR:

• Introduce prost dependencies
• Added build.rs to (re)generate prost structs and their corresponding protobuf wrappers (thanks to @nrc!)
• The generated wrappers are manually edited a little bit, please don't run regenerate yourself ATM
• Passed all tests by changing direct usages of protobuf APIs into prost APIs

I suppose test-passing means ready-for-review, please leave your comments!

[ice1000]

The rest of the refactoring depends on tikv/protobuf-build#2

[nrc]

The rest of the refactoring depends on tikv/protobuf-build#2

For at least the proto which is exported via kvproto, the Message impl is used by TiKV, so I think we need to keep those Message impls around for now.

[ice1000]

That sounds reasonable. I'll add the protobuf stuff back.

[nrc]

Looks great, just two very minor things remaining

[ice1000]

Test failing because

No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.
Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received
The build has been terminated

I have no idea about this.

[nrc]

LGTM \o/

[nrc]

Travis is failing on Windows only, @Hoverbear can we ignore that?

[Hoverbear]

Seems the vast majority of these changes are fairly mechanical. After chatting a bit and investigating the situation around new_() (which I do not like!) and how we'll fix the situation, I feel this is a good step of progress to merge.

I would prefer if we rapidly move to a state where we have functions without this naming convention. Preferably before 0.6.0

[Hoverbear]

@nrc We cannot! But this failure is related to travis, not you. We can force a few rebuilds and it should be green. I'll shepherd it.