Message collapsing

[martinthomson]

Need to do this once we have #3 and #6 are sorted out.

[brianraymor]

If we decide to pursue - To keep related changes together, the pull request should include the changes for the conditional DELETE of the message resource by the user agent.

[martinthomson]

Defer to -01.

[martinthomson]

Defer again.

[brianraymor]

Deferred multiple times. Closing. Can be reopened if there is a requirement to support this scenario.

[martinthomson]

Discussed in Yokohama: action on @martinthomson to generate a pull request that uses a string collapse key. That would allow the application server to operate in a stateless mode, something that the previous pull request would not allow.

[martinthomson]

Just did a scan of the header fields that are already registered. The only one that is even a remote possibility for this purpose is SLUG. It looks like we might want to ask for another new one, current candidate: Topic.

[brianraymor]

Topic sounds like a better option than reuse in this case.

[costinm]

"Topic" may be a bit confusing, it is commonly used in pubsub. I don't know if pubsub uses a Topic header, but it may still confuse developers.

[RichardMaher]

So this is where it all went wrong :-(

#87

_TOPICS are NOT collapse keys!_