INT-2856:Add support for adding/removing individual recipients to the Re...

Add support for adding/removing individual recipients to the RecipientListRouter
What is a `public` goal of this getter? I'd prefer do not expose them at all to avoid undesired mutation.
I understand the point for `expression` comparison, but it seems for me we have to encapsulte `equals` logic to the `Recipient` class. I think it doesn't hurt to have an extra GC from the `removeRecipient(String channelName, String selector)`. Or from other side we should check the type of `MessageSelector` to get access to its `expressionString`