Add a way to re-execute a query

Unless I'm mistaken, there is currently no way to re-execute an executed search query (other than manually passing it to the search server, I guess). We should therefore add a way to do that. I see several options:

1. Provide something like a reexecute() method (or maybe just a $force flag to execute()?), or a method to reset the query to its unexecuted state. With this approach, the results of the original query would have to be removed from the query object and therefore get lost.
2. Provide a method for getting an unexecuted clone of a query.
3. Just make the built-in clone default to producing an unexecuted query. I can't really think of a use case for cloning a query along with its result set – so this might be OK? It would change current behavior, though, so probably not a good idea to do at this stage.

In all those cases, we should probably take care not to run hooks and processors on the query twice so, when re-executing a query, preExecute() should probably not do anything. (I think. Not sure, though. Maybe leave that to callers via setProcessingLevel()?)