ip_mroute: refactor bw_meter API

ip_mroute: refactor bw_meter API

API should work as following:
- periodicaly report Lower-or-EQual bandwidth (LEQ) connections
  over kernel socket, if user application registered for such
  per-flow notifications
- report Grater-or-EQual (GEQ) bandwidth as soon as it reaches
  specified value in configured time window

Custom implementation of callouts was removed. There is no
point of doing calout-wheel here as generic callouts are
doing exactly the same. The performance is not critical
for such reporting, so the biggest concern should be
to have a code which can be easily maintained.

This is ia preparation for locking rework which is highly inefficient.

Approved by:    mw
Sponsored by:   Stormshield
Obtained from:  Semihalf
Differential Revision:  https://reviews.freebsd.org/D30210