Cache sphinx search results (specially when faceted search support is enabled)

When faceted search is enabled, users may navigate from results page to results page while changing filters from the guided search block.

At this moment there is no caching method for search results, being part of the reasons the fact that Sphinx is really fast resolving search queries (often just a few milliseconds).

However, many users start to use guided search this might generate a bit of contention on the server where the search daemon is running. Well, I think that could possibly happen, so maybe we need to implement some kind of search results caching to prevent that.

That being said, thoughts/questions/doubts:
- Where could we store cached results that could run faster than simply passing all queries to Sphinx?
- If we do this using our cache table, then we could combine this with Cache Router, Memcached, etc.
- We would have to measure different methods.
- Anything else?