MINOR: sink: add processed events counter in sft

MINOR: sink: add processed events counter in sft

Add a new struct member to sft structure named e_processed in order to
track the total number of events processed by sft applets.

sink_forward_oc_io_handler() and sink_forward_io_handler() now make use
of ring_dispatch_messages() optional value added in the previous commit
in order to increase the number of processed events.