Default brute-force query algorithm. Works by hoisting regular expressions
out of the query tree to narrow down the set of topics to be tested. Then
uses the query 'evaluate' method on each topic in turn to fully evaluate
the remaining query.
Not sure exactly where the breakpoint is between the
costs of hoisting and the advantages of hoisting. Benchmarks suggest
that it's around 6 topics, though this may vary depending on disk
speed and memory size. It also depends on the complexity of the query.