NonBlockingClient.send will create a Link for each unique topic, can't release them

[leachbj]

Each call to a send() method with a unique topic will result in a new Link back to the broker however there is no automatic cleanup for old/unused Link's nor any api to trigger. This results in a build up of Sender instances on the connection.

[dnwe]

@leachbj are you happy that the SendOptions.retainLink from pull #18 sufficiently addresses this issue? Or do you think we should also add automatic cleanup of stale links?

[leachbj]

The manual mechanism should be fine for most cases. It probably makes sense to have some clean up mechanism but not sure what the exact API/mechanism should be. So just something to think about for the future.