crimson/os/seastore/btree_types: BtreeCursors don't hold local copies of

crimson/os/seastore/btree_types: BtreeCursors don't hold local copies of
lba/backref values

Since lba mapping values might change during the executions of
client transactions once we allow background transactions to be
submitted without invalidating client ones, we want to avoid other
components using lba/backref mappings from keep local copies to prevent
petential problem

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