Replace rmlock with epoch in lagg

Use the new epoch based reclamation API. Now the hot paths will not
block at all, and the sx lock is used for the softc data. This fixes LORs
reported where the rwlock was obtained when the sxlock was held.

Submitted by: mmacy
Reported by: Harry Schmalzbauer <freebsd@omnilan.de>
Reviewed by: sbruno
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D15355