Make `cluster.addMultipleTargetTopics` _slightly_ more effecient

[alphashuro]

Is your feature request related to a problem? Please describe.
I'm seeing a lot of timeout acquiring lock errors in a project I inherited, and tracked it down to repeated calls to send causing the mutatingTargetTopics lock to be waited on many times. This is unexpected, as the number of target topics actually used is quite low, and I would not expect the targetTopics to be mutated if they have not actually changed.

Describe the solution you'd like
I would like to have the lock skipped entirely unless the target topics have actually changed.

I have implemented this in a draft PR here.