Change notification publisher API

Current API provides blocking and nonblocking .offer methods.

For better control over notification processing state it would be useful to get future object holding notification enqueue result.

Affected use case:
openflowplugin needs to deliver as much packetIn notifications into md-sal as possible. In case the top notification throughput is reached we need to apply backpressure to ingress. And also as soon as the load in md-sal notification pipeline gets back then the backpressure mechanism should be stopped.