Protect global ifnet list and short lived ifaddr references with epoch

[mmacy]

Summary

My weekend project has been improving UDP transmit performance:
https://github.com/ScaleBSD/scalebsd.github.io/blob/master/_drafts/2018-05-12-UDP-On-FreeBSD-is-anyone-using-this-at-scale.md

• To address the contention on ifnet_rwlock seen in the second flame graph I replaced rlock calls with epoch_{enter, exit} and made the necessary changes to ensure a grace period elapsed on deletion of ifnet and other things whose liveness is protected by said lock.

• To address the contention on the ifaddr reference that that in turn revealed I ensured that all short term references to an ifaddr were instead protected by epoch. Refcount updates are now only needed for persistent references on the heap.

Test Plan

pho and canary at Limelight Networks

[mmacy]

• update to today's HEAD
• merge with D15410: replace AF_DATA rlock path with epoch

[mmacy]

I think a week is a sufficient comment period for a straightforward change. I'll be committing this on Monday if there are no issues seen by pho or LLNW.