kvproto should not depend on tipb

[siddontang]

https://github.com/pingcap/kvproto/blob/master/proto/coprocessor.proto#L28

As we can see, kvproto uses tipb library, but it is not suitable here. They shouldn't depend on each other.

Because here we only need a Unmarhaler struct, maybe we can copy the struct directly to allow redundant code.

/cc @disksing