TopologyDescription::clone should make a deep copy of the ServerDescriptions

Task: Before associating the server descriptions to the cloned TopologyDescription, the result (cloned) TopologyDescription’s _servers should deep copied.

Context: TopologyDescription::clone currently makes a shallow copy of the source TopologyDescription. The result’s _servers point to the same underlying ServerDescriptions as the source’s _servers. This becomes an issue since the method mutates the underlying _servers for the source TopologyDescription as well as the result (cloned) version - causing unexpected changes when readers (primarily the ServerSelector) expect the TopologyDescription to be readonly.