public ApnsServiceBuilder asPool(ExecutorService executor, int maxConnections) {
        this.pooledMax = maxConnections;
        this.executor = executor;
        return this;
    }
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.

Note: The maxConnections here is used as a hint to how many connections
get created.
src/main/java/com/notnoop/apns/ApnsServiceBuilder.java