Match Bitcoin Core utxo serialization

[dergoegge]

We should change the way we serialize utxos to match Bitcoin Core:

https://github.com/bitcoin/bitcoin/blob/398dd678338971d2189934713c83c184742f293f/src/coins.h#L60-L65

vs ours:

utreexo/btcacc/leaf.go

Lines 75 to 82 in 2faac6f

	err = binary.Write(w, binary.BigEndian, hcb)
	err = binary.Write(w, binary.BigEndian, l.Amt)
	if len(l.PkScript) > 10000 {
	err = fmt.Errorf("pksize too long")
	return
	}
	err = binary.Write(w, binary.BigEndian, uint16(len(l.PkScript)))
	_, err = w.Write(l.PkScript)

This has several benfits:

• its easier to make the utcd node and Bitcoin Core bridge i am working on compatible.
• saves bandwidth because of the compression that core uses

[kcalvinalvin]

Sounds good but that does require varint encoding and unfortunately we can't import from utcd/btcd because that results in import cycles.

Two ways of going about this:

1: Move package btcacc/ into utcd/btcd
2: Copy over varint code from utcd/btcd

Requesting input from @adiabat