Previously we used pointer, this could have worked,
because most of the time, the netmap is cached.
This didn't work, however, because `lastNm` field was always nil.
Rework the mechanism completely:
1. Use epoch to track netmap versions, as it it simpler and
is unrelated to the TTL of an underlying cache.
2. Fix a bug where the epoch could change while mutex was unlocked.
Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>
