Migrates the p2p connections to Protobuf. Supersedes #4800.
gogoproto's `NewDelimitedReader()` uses an internal buffer, which makes it unsuitable for reading individual messages from a shared reader (since any remaining data in the buffer will be discarded). We therefore add a new `protoio` package with an unbuffered `NewDelimitedReader()`. Additionally, the `NewDelimitedWriter()` returns the number of bytes written, and we've added `MarshalDelimited()` and `UnmarshalDelimited()`, to ease migration of existing code.
