[quicwg/base-drafts] Define INITIAL_PRIORITY frame and remove exclusive dependencies (#2075)

[Mike Bishop <[e-mail]>]

Fixes #1865.  The observations from the issue discussion are:
- HTTP/2's "exclusive" prioritization (which make the prioritized element the sole child of the parent, making all current parents children of the prioritized element instead) is the cause of many ordering headaches and actually makes it impossible to guarantee an eventually-consistent view of priority between client and server.
- If a PRIORITY frame gets delayed and a request is quickly processed, it may be processed immediately (default is pretty high) and only later be discovered to be unimportant.

In the absence of exclusive dependencies, it's possible for a request stream to carry its own initial priority, as @dtikhonov proposed.  This is guaranteed to be the first time that a priority is assigned to this request, and subsequent PRIORITY frames always override it.  (If a PRIORITY frame gets reordered ahead, INITIAL_PRIORITY is ignored.)

This is a further departure from RFC 7540's priority scheme, but as it appears exclusive priorities and reordering are somewhat incompatible, I believe is still within our design space.

**Depends on #2074; retarget before merging.**
You can view, comment on, or merge this pull request online at:

  https://github.com/quicwg/base-drafts/pull/2075

-- Commit Summary --

  * INITIAL_PRIORITY

-- File Changes --

    M draft-ietf-quic-http.md (87)

-- Patch Links --

https://github.com/quicwg/base-drafts/pull/2075.patch
https://github.com/quicwg/base-drafts/pull/2075.diff

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