query cache should allow paginated listing by type

[britram]

Right now, GET /query yields an undifferentiated list of queries without any metadata. This is not terribly useful. Suggest a refactor to the following:

• GET /query/pending -- links to currently pending queries (either submitted, or not yet executing)
• GET /query/cached -- links to temporarily cached queries (complete, but without _ext_ref)
• GET /query/permanent -- links to permanently cached queries (complete and referenced)

Since this implies that the query cache will need to keep basically everything in memory anyway, these should probably be listed with at least their encoded form, so a single front-end query can provide a useful listing

(tl;dr, this is pretty much what pto2.5 does)