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.