Currently the session pool suggests `end_sessions` method as the way of clearing itself, however this method's exception handler can stop execution prior to removing all sessions from the pool:

      def end_sessions
        while !@queue.empty?
          server = ServerSelector.get(mode: :primary_preferred).select_server(@cluster)
          Operation::Command.new(
              :selector => {endSessions: @queue.shift(10_000).collect { |s| s.session_id }},
              :db_name => Database::ADMIN).execute(server)
        end
      rescue
      end

If the pool contains more than 10,000 sessions and there is an error removing the first 10,000, subsequent sessions won't be removed. If there is a problem selecting a server no sessions will be removed.

There should be a way to clear the pool client side even if sessions cannot be killed on the server.