event/sw: simplify credit scheme
event/sw: simplify credit scheme

This commit modifies the sw PMD credit scheme such that credits are
consumed when enqueueing a NEW event and released when an event is
released -- typically, the beginning and end of a pipeline. Workers that
simply forward events do not interact with the credit pool.

Signed-off-by: Gage Eads <40e74f4682b7928915cbf15b25c304eff85f59a4@intel.com>
Acked-by: Harry van Haaren <b9ad04ed670c22cfa7f5f9ab17e311ca85d11a76@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk