when tracking the peak, don't reset the peak to 0, reset it to the
maximum of the current used, and the planned to be used by the current
arg.

when shrining, split the two separate conditions.
the idle time shrinking will remove all free space.
but the peak based shrinking will keep room for the current arg.

when we resize due to a peak (rahter than idle time), don't trim all
unused space, let the qbuf keep a size that's sufficient for the
currently process bulklen, and the current peak.
Co-authored-by: Nsundb <sundbcn@gmail.com>
Co-authored-by: Nyoav-steinberg <yoav@monfort.co.il>