crimson/os/seastore/cache: add an efficient method to check if extents are

crimson/os/seastore/cache: add an efficient method to check if extents are
viewable to transactions

Instead of searching the transaction's retired_set to determine whether
an extent has been retired, we add the transaction that's retiring an
extent to that extent's retired_transactions field and search that field
to do the check. Since the probability of multiple transactions retiring
the same extent is very low, this approach should be more cpu efficient.

Signed-off-by: Xuehan Xu <xuxuehan@qianxin.com>