Run the bridge datapath under epoch, rather than under the
We still take the BRIDGE_LOCK() whenever we insert or delete items in
the relevant lists, but we use epoch callbacks to free items so that
it's safe to iterate the lists without the BRIDGE_LOCK.
Tests on mercat5/6 shows this increases bridge throughput significantly,
from 3.7Mpps to 18.6Mpps.
Sponsored by: The FreeBSD Foundation