New proactor APIs to query event batch for subject

The object that a proactor event batch refers can't change during the processing of the batch so you shouldn't need to extract the object from every individual event of the batch,

This change introduces an API for each of the possible subjects of an event batch:
pn_event_batch_proactor(), pn_event_batch_listener(), pn_event_batch_connection(), pn_event_batch_raw_connection(),