public ApnsServiceBuilder asPool(int maxConnections) {
        return asPool(Executors.newFixedThreadPool(maxConnections), maxConnections);
    }
Constructs a pool of connections to the notification servers.

Apple servers recommend using a pooled connection up to
15 concurrent persistent connections to the gateways.

Note: This option has no effect when using non-blocking
connections.
src/main/java/com/notnoop/apns/ApnsServiceBuilder.java