crimson/osd: use app.alien() to initialize AlienStore::alien

crimson/osd: use app.alien() to initialize AlienStore::alien

in e53ea0886fba9073904f59ea85fb73d854565921, the new alien::submit_to() API
is used in the place of the old one. but seastar::alien::instance::_qs
should be initialized before we are able to use the alien instance, just
creating an instance of alien::instance is not enough.

in this change, instead of creating an instance of alien::instance using
make_unique<>, the return value of app.alien() is used to initialize the
alien member variable of AlienStore. app.alien() is always properly
initialized by reactor.

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