Adding the available_queries_for_explain thread local into the
ActiveRecord runtime registry to try to manage thread locals.
Added tests for the runtime registry making sure that the correct save
and restore behavior occurs, as well as the correct behavior regarding
arrays for the available_queries_for_explain attribute.
Adding a means for saving available queries on the runtime registry.
This sets the collecting_queries_flag to true, and allows the finish
method to run correctly in the explain_subscriber.