flow-manager: reduce burstiness in adaptive timing

flow-manager: reduce burstiness in adaptive timing

Previous adaptive model would have a large time range when scanning the
hash when not so busy. In the default case it would take up to 4 minutes
for a full hash scan. In case of sudden increase in business, where the
hash would fill up rapidily during a few seconds, the flow manager would
be forced to suddenly consider a much larger slice of the hash leading
to a burst of work. This burst would increase pressure on the rest of the
system leading to packet loss as the worker threads would be overloaded
with flow housekeeping tasks.

This patch reduces the max scan time to 10 seconds, and ramps up quickly
to increase the slice of the hash scanned.