librados: fix common_init_finish timing

librados: fix common_init_finish timing

Common_init_finish does start_service_thread and does
set_safe_to_start_threads() on the cct, which switches us to 'runtime'
mode where we can't accept many config options. Do that *after* we fetch
our config from the mon so that we can accept+set runtime options (and
not complain to stderr about it).

Signed-off-by: Sage Weil <sage@redhat.com>