Two options control that queue: a maximum size ([PoolingOptions.setMaxQueueSize][smqs]) and a timeout

([PoolingOptions.setPoolTimeoutMillis][sptm]).

* if either option is set to zero, the attempt is rejected immediately;

* else if more than `maxQueueSize` requests are already waiting for a connection, the attempt is also rejected;

* otherwise, the attempt is enqueued; if a connection becomes available before `poolTimeoutMillis` has elapsed,