fix(sessions): never send `endSessions` from a `ClientSession`
fix(sessions): never send `endSessions` from a `ClientSession`

Sessions are meant to be cached, and the `endSessions` command is
only meant to be used as a way to signal to the server that cached
sessions will no longer be used (due to the client closing). We
were sending too many `endSessions` commands, now we just send the
one.

NODE-1418

mongodb-js/mongodb-core,mongodb-js/mongodb-core