Interactive performance regression since 2.0

[Matthew Dillon]

    Ok, I tracked the problem down.  Basically what is happening is that
    a new current user lwp is selected while other user threads are
    sitting in the LWKT run queue which have not yet had a chance to add
    themselves back to the userland queue.

    The problem is that I can't just cycle the run queue by calling
    lwkt_switch(), because new user threads may pop onto it at any time.

    I hope to be able to fix it tonight but it will require some
    significant work and may extend into tomorrow.

						-Matt