Use vtprotobuf for faster unmarshalling and less mallocs

[krdln]

Links:

• https://vitess.io/blog/2021-06-03-a-new-protobuf-generator-for-go/
• https://buf.build/community/planetscale-vtprotobuf

Performance improvements should come from two aspects:

• vtprotobuf avoids reflect and generate specialized code,
• vtprotobuf allows pooling object to reduce allocs.

[krdln]

Discovered that pooling in vtprotobuf is a bit underpowered (this is known – there are issues on the repo):

• doesn't work for oneofs,
• doesn't work for slices of messages,
• doesn't work for maps.

Basically, in practice pooling basically works for preserving capacity for slices of strings, which is something, but leaves lot of allocs on the table. Also, note that the major source of allocs in proto unmarshalling are strings, which is something not fixable by pools in Go.