Convert if_afdata read lock path to epoch

[mmacy]

Summary

• Convert write lock to mutex
• Convert read lock to epoch
• Do destroy / free of llentrys after grace period has elapsed

For the impact see flame graph 4 -> 5 at:
https://github.com/ScaleBSD/scalebsd.github.io/blob/master/_drafts/2018-05-12-UDP-On-FreeBSD-is-anyone-using-this-at-scale.md