We've yet to really wire up the eventual consistency of btree ring
blocks and segments. The btree block reading code has had a warning
that fires if it sees stale blocks for a long time (which we've yet to
hit) but we have no such warning in the segment. If we hit stale
segments we could have very unpredictable results. So let's add a quick
warning to highlight the case to save us heartache if we hit it before
implementing full retrying.
Signed-off-by: Zach Brown <zab@versity.com>