Combine preferences callback and track-changes on defining query

[ralfhandl]

Current specification text suggests that preference callback can only be used

• on a GET request to a delta link

This suggests that two requests are necessary for being notified of changes:

• Send defining query with preference track-changes
• Send GET request to delta link with preference callback

Allow combining this into one request:

• Send defining query with both preference track-changes and preference callback

[HeikoTheissen]

It seems problematic that callback is a preference on its own which can appear together with another preference, either respond-async or track-changes. If callback was an attribute of these other preferences, it could appear in both in one request.

[ralfhandl]

Extend current state

Prefer: track-changes

• return delta link for polling

Prefer: track-changes,callback;url=http://me

• return delta link, callback on changes

Prefer: track-changes;mode=no-delta-link

• don't return delta link, kind of useless

Prefer: track-changes;mode=no-delta-link,callback;url=http://me

• don't return delta link, callback on changes

Problem: can't prefer to have a complicated defining request processed async and get a callback for the async processing and a different callback for the available delta

Add callback-url attribute to other preferences, optionally deprecate callback preference in combinat

Prefer: track-changes;callback-url=http://me

• return delta link, callback on changes

Prefer: track-changes;no-delta

• don't return delta link, kind of useless

Prefer: track-changes;no-delta;callback-url=http://me

• don't return delta link, callback on changes, solves Introduce a method for fully automated data replication of small data sets #1962

GET /complicated-collection
Prefer: respond-async;callback-url=http://me/initial,track-changes;callback-url=http://me/delta