vinyl: allow to skip prepared statements in cache iterator

To implement read-confirmed and best-effort isolation levels, we need
to skip unconfirmed (aka prepared) statements in the cache iterator. To
achieve that, we add a new flag is_prepared_ok. Unless the flag is set,
the iterator will skip prepared statements even if they are visible from
the iterator read view. Note, in contrast to the mem iterator, we don't
need to keep track of the min skipped statement LSN, because the cache
is just a view of the underlying levels so we'll find it out when we
descend to the mem level.

Needed for #5522

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