CDRIVER-2238 race in mongoc_client_kill_cursor
CDRIVER-2238 race in mongoc_client_kill_cursor

Once the topology mutex is unlocked, it isn't safe to use a server
description from the topology: the server description could be freed by
a topology change before mongoc_client_kill_cursor accesses its server
id. I had apparently begun to make this thread-safe a while ago but
didn't finish the job until now.

derickr/mongo-c-driver,rcsanchez97/mongo-c-driver,acmorrow/mongo-c-driver,beingmeta/mongo-c-driver,derickr/mongo-c-driver,acmorrow/mongo-c-driver,beingmeta/mongo-c-driver,beingmeta/mongo-c-driver,acmorrow/mongo-c-driver,beingmeta/mongo-c-driver,mongodb/mongo-c-driver,mongodb/mongo-c-driver,derickr/mongo-c-driver,ajdavis/mongo-c-driver,jmikola/mongo-c-driver,rcsanchez97/mongo-c-driver,remicollet/mongo-c-driver,mongodb/mongo-c-driver,ajdavis/mongo-c-driver,bjori/mongo-c-driver,beingmeta/mongo-c-driver,acmorrow/mongo-c-driver,jmikola/mongo-c-driver,acmorrow/mongo-c-driver,derickr/mongo-c-driver,mongodb/mongo-c-driver,bjori/mongo-c-driver,rcsanchez97/mongo-c-driver,beingmeta/mongo-c-driver,remicollet/mongo-c-driver,rcsanchez97/mongo-c-driver,bjori/mongo-c-driver,ajdavis/mongo-c-driver,acmorrow/mongo-c-driver,bjori/mongo-c-driver,rcsanchez97/mongo-c-driver,mongodb/mongo-c-driver,jmikola/mongo-c-driver,derickr/mongo-c-driver,bjori/mongo-c-driver,beingmeta/mongo-c-driver,rcsanchez97/mongo-c-driver,ajdavis/mongo-c-driver,remicollet/mongo-c-driver,remicollet/mongo-c-driver,remicollet/mongo-c-driver,remicollet/mongo-c-driver,ajdavis/mongo-c-driver,mongodb/mongo-c-driver,remicollet/mongo-c-driver,jmikola/mongo-c-driver,bjori/mongo-c-driver,ajdavis/mongo-c-driver,mongodb/mongo-c-driver,ajdavis/mongo-c-driver,acmorrow/mongo-c-driver,jmikola/mongo-c-driver,jmikola/mongo-c-driver,beingmeta/mongo-c-driver,jmikola/mongo-c-driver,bjori/mongo-c-driver,derickr/mongo-c-driver,rcsanchez97/mongo-c-driver,derickr/mongo-c-driver