service: add function to run on app lcore

service: add function to run on app lcore

This commit adds a new function which allows an application lcore
(aka; *not* a dedicated service-lcore) to run a service. This
function is required to allow applications to gradually port
functionality to using services, while still being able to run
ordinary application functions.

This requirement became clear when a patch to the existing
eventdev/pipeline sample app was modified to use a service-core
for scheduling - and that same core should be capable of running
the "worker()" function from the application too.

This patch refactors the existing running code into a smaller
"service_run" function, which can be called by the dedicated
service-core loop, and the newly added function.

[1] http://dpdk.org/ml/archives/dev/2017-October/079876.html

Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
Acked-by: Pavan Nikhilesh <pbhagavatula@caviumnetworks.com>