#29 persist results to minimize memory usage

[Sven Schrecker]

When a particular results tab is not "on top", the
application should persist the results and free the
memory associated with the results.

There could be a configurable memory limit that
allows some number of queries/results to stay
resident in memory, but when the tab drops from
the queue, it is persisted. This allows for a much
smaller memory footprint while maintaining the
flexibility of multiple open result sets.