Modify backfill algorithm

Modify backfill algorithm to improve performance with large numbers of
    running jobs. Group running jobs that end in a "similar" time frame using a
    time window that grows exponentially rather than linearly. The original
    window sizes were (in units of minutes):
    0, 1, 2, 3, 4, 5, 6, 7, ... minutes
    The new window sizes are:
    0.5, 1, 2, 4, 8, 16, 32, ... minutes
    This can dramatically reduce the number of instances where the very time
    consuming "can the pending job run now" operation is executed, especailly
    if there are 1000+ running jobs.
bug 3275