transport-30 introduces a stealthy new MUST

[Dmitri Tikhonov]

Hello,

There is a new paragraph in transport-30:

   Prior to receiving an acknowledgement for a packet number space, the
   full packet number MUST be included.

This requirement was added in 35b28e13aa41ebc53b3e053a8b52868bfb81a8e8,
while the actual "MUST" was added in f009224fcd784f2e5ea88bdb11dcdb4adfb0badd

I have two problems with this change:

    1. The new requirement does not seem to have had a corresponding
        GitHub issue where this design change(?) would have been discussed.

    2. How can only include the *full* packet number when the header only
        allows a maximum of four bytes for this value, which can reach
        2^62 - 1?

  - Dmitri.