Clarify requirement to avoid wrapping CID seqenunce number

[gloinul]

Sectionm 5.2:
"Section 19 of [QUIC-TRANSPORT] encodes the Connection ID Sequence Number as a variable-length integer, allowing values up to 2^62-1; in this specification, a range of less than 2^32-1 values MUST be used before updating the packet protection key"

I think this is unlikely to cause a real issue, but is sloppy writing. At no point in time may a sequence number n and its next wrapped value ( n+2^32) be protected by the same packet protection key. This is unlikely to ever happen due to AEAD limits of at least current ciphers. But I think it would be good to be formally correct here.

I will note that there is duplication of the imprecise requirement in the last paragraph of the seciton too: "Due to the way the nonce is constructed, endpoints MUST NOT use more than 2^32 Connection IDs without a key update."

[Yanmei-Liu]

I think we have consensus here to limit the upper bound of Path ID to 2^32-1 and avoid reuse of nonce here.
I've updated the description in PR #292.

[mirjak]

Before I close this issue:

The text says:

the Path ID is an integer between 0 and 2^32 - 1 (inclusive).

Do we need normative language here? Also do we need to specify what to do if a larger path ID is sent? Can that even happen?