Move ENA's hw stats updating routine to separate task

Initially, stats were being updated each time OS was requesting for
the first statistic.
To read statistics from hw, condvar was used. cv_timedwait cannot be
called when unsleepable lock is held, and this happens when FreeBSD
is requesting statistic.
Seperate task is reading statistics from NIC each 1 second.

Submitted by: Michal Krawczyk <mk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Amazon.com Inc.
Differential revision: https://reviews.freebsd.org/D10926