Consider whether RIB callbacks should be async/channel writes.

[robshakir]

Today, we make a callback to the post-change function in the RIB when an operation has completed, this is currently blocking.

As @alshabib rightly points out -- this could cause performance issues. We should consider one of two things:

• calling the callback in a goroutine so that it doesn't block.
• deprecating the callback approach, and rather just providing a channel that we write these events to that the consumer can listen on