agreement: avoid re-encoding proposal messages

[cce]

Draft of avoiding re-encoding proposals when relaying them, by keeping references to the original encoded transmittedPayload message and the unauthenticatedVote contained inside it.

[ghost]

Do we need to check the proposal vote is correct? Assuming agreement isn't broken, the encoded message should always be the same.

[icorderi]

LGTM

[algorandskiy]

panic: msgpack decode error [pos 46]: cannot decode unsigned integer: unrecognized descriptor byte: a4/string|bytes

goroutine 255 [running]:
github.com/algorand/go-algorand/agreement.(*testingNetwork).multicast(0xc0001b01c0, {0x1377d11, 0x2}, {0xc003436700, 0x319, 0x688}, 0x0, 0x0)
	/opt/cibuild/project/agreement/service_test.go:210 +0x1585

		if tag == protocol.ProposalPayloadTag {
			r := bytes.NewBuffer(data)

			var tp transmittedPayload
			err := protocol.DecodeStream(r, &tp)
			if err != nil {
				panic(err)
			}

Looks like it tries to decode pv field and gets only an encoded value and not encoded key-value pair?

[cce]

Can revisit later (would be good to also avoid re-compressing proposals when relaying them) after or as part of a consensus release