service: add mechanism for quiescing

service: add mechanism for quiescing

Existing service functions allow us to stop a service, but doing so doesn't
guarantee that the service has finished running on a service core. This
commit introduces rte_service_may_be_active(), which returns whether the
service may be executing on one or more lcores currently, or definitely is
not.

The service core layer supports this function by setting a flag when
a service core is going to execute a service, and unsetting the flag when
the core is no longer able to run the service (its runstate becomes stopped
or the lcore is no longer mapped).

With this new function, applications can set a service's runstate to
stopped, then poll rte_service_may_be_active() until it returns false. At
that point, the service is quiesced.

Signed-off-by: Gage Eads <gage.eads@intel.com>
Acked-by: Harry van Haaren <harry.van.haaren@intel.com>