MFC r357294:
Widen EPOCH(9) usage in mlx5en(4).

Make completion event path mostly lockless using EPOCH(9).

Implement a mechanism using EPOCH(9) which allows us to make
the callback path for completion events mostly lockless.

Simplify draining callback events using epoch_wait().

While at it make sure all receive completion callbacks are
covered by the network EPOCH(9), because this is required
when calling if_input() and ether_input() after r357012.

Sponsored by: Mellanox Technologies