libceph: schedule tick from ceph_osdc_init()

Both homeless OSD sessions and watch/notify v2, introduced in later
commits, require periodic ticks which don't depend on ->num_requests.
Schedule the initial tick from ceph_osdc_init() and reschedule from
handle_timeout() unconditionally.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>