[HER-1714] fixed-interval rescheduling

* AbstractFrontier.java, WorkQueueFrontier.java, BdbFrontier.java
    add 'futureUris' map of CrawlURIs to be reenqueued at specific future dates
    the usual findEligibleUri also checks for newly-eligible rescheduled URIs
    the frontier is no longer empty unless the future count is also 0
* CrawlURI.java
    add rescheduleTime property
* ReschedulingProcessor.java
    optional post-processor to set a CrawlURI's reschedulingTime
* CrawlStatSnapshot.java, CrawlJob.java
    add futureUriCount to stats/uriTotalsReport

modules/src/main/java/org/archive/modules/CrawlURI.java

@@ -1826,4 +1826,15 @@ implements MultiReporter, Serializable, OverlayContext {
     public CrawlURI getFullVia() {
         return fullVia;
     }
+
+    /**
+     * A future time at which this CrawlURI should be reenqueued.
+     */
+    protected long rescheduleTime = -1;
+    public void setRescheduleTime(long time) {
+        this.rescheduleTime = time;
+    }
+    public long getRescheduleTime() {
+        return this.rescheduleTime;
+    }
 }