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
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