statd: increase RSS difference required for restart

statd: increase RSS difference required for restart

it seems that we have a reference leak or the like somewhere in the
(graphite?) status plugin, while the recent transaction based update
mechanism made it slightly better, it's still bad with a lot of VMs..

Until we can track that down, or abandon perl for good, avoid to
frequent restarts by allowing statd to grow 15 MB of memory usage
after initial calibration (it's memory usage at the 10th cycle)

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>