crimson/seastore: fix reactor stalled and rbd_open failed

crimson/seastore: fix reactor stalled and rbd_open failed

omap manger use keys' reference, so the keys' string set should
be kept alive during omap operation. use value capture instead of
reference capture.

otherwise, during omap operation, the keys contents will be changed
and cause logger insert_iterator very long time to stall reactor.
omap_get_value also failed, rbd_open failed.

Signed-off-by: chunmei-liu <chunmei.liu@intel.com>