Design Issue: Separate HEADERS and PRIORITY Frames, Eliminate HEADERS+PRIORITY

[James M Snell <[e-mail]>]

https://github.com/http2/http2-spec/issues/99

With regards to the discussion over stream re-prioritization, I suggest:

1. Drop the HEADERS+PRIORITY frame type.
2. Create a new separate PRIORITY frame type whose payload is the
Priority value, no frame-specific flags.
3. The PRIORITY frame becomes the only way to set/change the priority
for a stream.

If it is necessary to allow an endpoint to establish the priority of
stream prior to actually initiating the stream, we can allow sending a
PRIORITY frame before the initial HEADERS frame. Doing so would
effectively reserve the stream id (in the same general manner
PUSH_PROMISE does).

The advantages of this approach are:

1. It eliminates any possible confusion and complexity about when to
use HEADERS+PRIORITY vs. HEADERS
2. It provides a single way of setting/change stream priority (as
opposed to using HEADERS+PRIORITY plus a separate CHANGE-PRIORITY
frame)