Use single layer of messages

[cthulhu-rider]

In current implementation we have 2 layers of messages:

• so-called v2 layer with types providing stable marshaling
• grpc layer with auto-generated types

After #389 generated messages provide stable marshaling, so we can purge v2 layer on type-level.

We also need to adopt rpc functionality.

[fyrchik]

1. This way we could lose the benefit from our previous optimizations Get rid of the overhead use of pointers in field setters and repeated fields #376 .
2. Because now SDK uses v2 aliases in many places, we would need to implement similar wrappers in SDK. They can be more optimal there, though.