crimson/os/seastore: use map::merge() to merge maps

crimson/os/seastore: use map::merge() to merge maps

C++17's std::map allows us to merge two maps, and in this case, we can
even consume `child_result`. so map::merge() is used instead of insert()
in hope to avoid the memcpy and allocation of pair<> nodes.

Signed-off-by: Kefu Chai <kchai@redhat.com>