Add an option to limit the number of concurrent queries.
Add an option to limit the number of concurrent queries.

paulproteus/livegrep,lekkas/livegrep,lekkas/livegrep,lekkas/livegrep,paulproteus/livegrep,wfxiang08/livegrep,lekkas/livegrep,wfxiang08/livegrep,paulproteus/livegrep,lekkas/livegrep,paulproteus/livegrep,paulproteus/livegrep,wfxiang08/livegrep,wfxiang08/livegrep,lekkas/livegrep,paulproteus/livegrep,wfxiang08/livegrep,wfxiang08/livegrep