ring: introduce peek style API

ring: introduce peek style API

For rings with producer/consumer in RTE_RING_SYNC_ST, RTE_RING_SYNC_MT_HTS
mode, provide an ability to split enqueue/dequeue operation
into two phases:
      - enqueue/dequeue start
      - enqueue/dequeue finish
That allows user to inspect objects in the ring without removing
them from it (aka MT safe peek).

Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>