Improve performance of the Tracker search engine

Modernize the Tracker search engine code to the 2019's by using TrackerSparqlStatement and caching for multiple query executions, and identify/fix several bottlenecks that were making this engine unnecessarily slow.

On a modern computer, and a Tracker database with ~1.4 million indexed files, search queries can now return elements within milliseconds, instead of seconds to minutes.

Since "spamming" search engines is no longer much of a concern, this MR also removes the timeout artificially delaying search.