CDRIVER-3645 Close operation connections to removed servers (#1618)

* add regression test: check connection counts of servers removed by SRV polling

* prune clients on push

Track last known server IDs in the client pool. Before doing an expensive prune, do a cheaper check to see if server IDs changed.

* test direct topology changes

Servers may be removed from the topology in other scenarios aside from SRV polling (e.g. during SDAM)

* document expected use of pop/push

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Co-authored-by: Ezra Chung <88335979+eramongodb@users.noreply.github.com>