mv value needs encoding specified

[mfoltzgoogle]

The mv value in the mDNS TXT record is an integer, but the spec does not say how to encode it.

Choices include:

1. ASCII as hex or base10
2. Big endian (network byte order)
3. QUIC varint

Since values are likely to remain small (< 10000), I would favor option 1 which prioritizes readability.

[chrisn]

I agree, and this would be consistent as the other values in the TXT record are also readable.

As an aside, reading this part of the OSP spec, there's a reference to RFC5122 for the fp field - should this be RFC8122?

[mfoltzgoogle]

Addressed by #253.