protocols/plaintext: Incompatible UVI encoding with Go implementation

[mxinden]

The Golang /plaintext/2.0.0 implementation uses UVI encoding for the handshake messages only, not for the actual goodput of the protocol.

https://github.com/libp2p/go-libp2p-core/blob/master/sec/insecure/insecure.go

The Rust /plaintext/2.0.0 implementation uses UVI encoding for both the handshake messages as well as all further messages exchanged over the protocol.

https://github.com/libp2p/rust-libp2p/blob/master/protocols/plaintext/src/lib.rs#L174

The /plaintext/2.0.0 specification is somewhat vague, saying all messages should be UVI encoded, but not whether that includes messages send after the handshake (goodput):

Message Framing

All messages sent over the wire are prefixed with the message length in bytes, encoded as an unsigned variable length integer as defined by the multiformats unsigned-varint spec.

https://github.com/libp2p/specs/tree/master/plaintext