jobstats: add abort function

jobstats: add abort function

This patch adds new function rte_jobstats_abort.
It marks *job* as finished and time of this work will be add to management
time instead of execution time.
This function should be used instead of rte_jobstats_finish if condition
occurs, condition is defined by the application for example when receiving
n>0 packets.
Example of usage is added to the example l2fwd-jobstats.
At maximum load do-while loop inside Idle job will be execute once because
one or more jobs waiting to be executed, so this time should not be include
as the execution time by calling rte_jobstats_abort().

Signed-off-by: Marcin Kerlin <marcinx.kerlin@intel.com>
Acked-by: Fan Zhang <roy.fan.zhang@intel.com>