crimson/os/alienstore: create tp in AlienStore::start()

crimson/os/alienstore: create tp in AlienStore::start()

thread pool is not needed until AlienStore::start(). with this change,
we are able to tell if the AlienStore is actually started or not in
AlienStore::stop().

as seastar::sharded<Service> start a service in two phases:

1. construct the shard instances
2. actually start them

and it stops a service in a single shot, which both stops the services
and destructs the service instance(s).

so we have to implement a proper stop() method for services whose
start() might not be called after its instance is created by
seastar::sharded<Service>::start() in case of error handling or if
we just don't want to call start().

to ensure we can skip the steps to clean up the stuff created by
start(), we need to have a flag in the sharded service, because
AlienStore is a member variable of OSD, and when we do mkfs, AlienStore
is not start()'ed, and as explained above, we have to call OSD::stop()
to ensure OSD instance is destructed properly. but OSD::stop()
calls store->umount() and store->stop() unconditionally. these methods
in AlienStore rely on a functional thread pool.

fortunately, we don't need to call these methods if the store is never
mounted or started. in a case of failed "mkfs", store is not mounted at
all but the store and osd instances are created.

so, in this change, thread pool is created in AlienStore::start(), and
we will use it to tell if AlienStore is started or not in the following
change which makes the related method no-op if AlienStore is not started
yet.

also, postpone the creation of `store` until in AlienStore::start(), so
we don't need to destroy it in the dtor of AlienStore. otherwise,
BlueStore::~BlueStore() would need to reference resources which are only
available in alien threads, but when OSD::~OSD() is called, we are in
seastar's reactor.

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