Switch serialization to protobuf & remove capn' proto

[appaquet]

capn' proto was chosen to prevent extra memory allocations in the chain, which turned to be complicated to achieve because of Rust's memory safety. Ex: there is no safe way right now to read a capn' proto message from an arbitrary bytes slice because of alignment concerns.

On top of that, for a proof of concept, using a single message serialization makes more sense.