public boolean hasPendingLinks() {
        return hasPendingLinksInFrontier() || scheduler.hasPendingLinks();
    }
This method loads links from the frontier (stored in disk) to the scheduler. The is scheduler
is a in-memory data structure that prioritizes links base on score and also politeness
constraints. Which links are selected to be inserted in the frontier is determined the policy
implemented by the LinkSelector configured.

@param numberOfLinks
ache/src/main/java/achecrawler/link/frontier/CrawlScheduler.java