Add an API for the common case

Add an API for the common case

Most of our current event listeners are single functions that don't
carry much state. Rather than force us to write a behavior module for
every one of these cases we can use this link_listener/4,
stop_listener/1 API to handle this common pattern.

Under the covers this just wraps couch_event_listener rather thinly to
give us the single callback export version of listening for events.