consumer: ability to disconnect from nsqd

this adds a function that allows the end-user to remove
addresses from the list of `nsqd` for situations where
they're not using `nsqlookupd` and instead maintaining
their own list.
mixed feelings as to weather or not this should error since we don't expose the list of where you are connected to. We don't typically expose named errors, but perhaps this would be a fair spot for `return NotConnectedError`.