crimson/osd: create store in main()

crimson/osd: create store in main()

to initialize an alienstore with a reference of
seastar::alien::instance is but the internal of AlienStore, would be
better if we could avoid exposing it as a part of the interface of
crimson::osd::OSD().

so, in this change, instead of creating FuturizedStore in OSD(), we
create it in main() where the app is available, so we can just create
FuturizedStore without passing the alien instance all the way down to
OSD().

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