[quicwg/base-drafts] Connection ID lengths 1, 2 and 3 bytes can't be encoded (#1570)

[Marten Seemann <[e-mail]>]

The connection IDs can be either 0 bytes or anything between 4 and 18 bytes (inclusive). Lengths of 1, 2 and 3 bytes are not possible.

There are several use cases that would benefit from using shorter connection IDs, especially in p2p applications. An endpoint could encode the length of the connection ID into the first few bits, and then starts using 1 byte connection IDs first, and successively moves to longer lengths only when handling more connections. For p2p hosts, in many cases 2 bytes would be sufficient for all use cases.

In the connection ID length field in the Long Header, we only have 4 bits to encode a range of 19 values (we want 0 byte connection IDs, and we want to support 18 byte connection IDs), so *some* values have to be left out. It seems there are few use cases for 13, 14 and 15 bytes.

We could achieve this by changing the definition of the length to:

>  An encoded length between 0 and 12 inclusive indicates that the connection ID is also 0 to 12 octets in length. Any larger value is increased by 3 to get the full length of the connection ID, producing a length between 16 and 18 octets inclusive. 

If people are supportive of this change, I'd be happy to write a PR.

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