Data cached by EnabledSourceHandler never expires

Been thinking on this a bit more, and the problem comes down to the fact that EnabledSourceHandler stands in deep violation of the single-responsibility principle. It does three things:

• It's a quasi-plugin manager, only letting us access enabled sources.
• It's a caching layer for query results.
• It's a stable, non-volatile repository of every project we have encountered while using Project Browser.

Fully cleaning this up is not in scope here. But I think we can create a cleaner separation between points #2 and #3.

Query results are just that: "when you give me these criteria, here are the projects I found". These should use a normal, volatile cache backend.

Stored projects are relied upon by the activators, so they cannot be subject to volatile caching. Clearing the stored projects needs to be -- and is -- a separate operation from querying.

So I think my proposed resolution is correct -- use the cache system for query results, and always update stored projects with whatever the source plugins return. Project object storage, though, should stay as it is, in non-expiring key-value stores.

Assigning to @tim.plunkett specifically, since this kind of stuff is very much in his wheelhouse.

This should be backported.

Decided to expand the scope here to encompass the full clean-up of EnabledSourceHandler into better-scoped, single-responsibility classes. Need to update the issue summary to reflect that.

Walked through this with @phenaproxima and asked for one additional code-comment, but this is great work!

!831 merged to 2.1.x

and !828 now in 2.0.x