[HER-1658] CachedBdbMaps not expunging as expected (especially StatisticsTracker.processedSeedsRecords)

* CachedBdbMap.java
    do expunge on put(), replace()
    add low-memory-sensitive 'canary' to force expunge even if otherwise untriggered
* CachedBdbMapTest.java
    add test of idle expunge in low-memory conditions
* CrawlURI.java
    (getURI) direct access to String URI (don't reuse toString() functionally)
* SeedRecord.java
    support for updating record with later/repeat report
* StatisticsTracker.java
    remove put()s for processedSeedRecords, hostsLastFinished
* BdbFrontier.java
    (getQueueFor) remove put, do in concurrent-compliant manner

commons/src/test/java/org/archive/util/CachedBdbMapTest.java

@@ -83,6 +83,32 @@ public class CachedBdbMapTest extends TmpDirTestCase {
         }
     }
     
+    /**
+     * Test that in scarce memory conditions, the memory map is 
+     * expunged of otherwise unreferenced entries as expected.
+     * 
+     * NOTE: this test may be especially fragile with regard to 
+     * GC/timing issues; relies on timely finalization, which is 
+     * never guaranteed by JVM/GC. 
+     * 
+     * @throws InterruptedException
+     */
+    public void testMemMapCleared() throws InterruptedException {
+        assertEquals(cache.memMap.size(), 0);
+        for(int i=0; i < 10000; i++) {
+            cache.putIfAbsent(""+i, new HashMap<String,String>());
+        }
+        assertEquals(cache.memMap.size(), 10000);
+        assertEquals(cache.size(), 10000);
+        TestUtils.forceScarceMemory();
+        Thread.sleep(1000);
+        // The 'canary' trick makes this explicit expunge, or
+        // an expunge triggered by a get() or put...(), unnecessary
+        // cache.expungeStaleEntries();
+        System.out.println(cache.size()+","+cache.memMap.size());
+        assertEquals(0, cache.memMap.size());
+    }
+    
     public static void main(String [] args) {
         junit.textui.TestRunner.run(CachedBdbMapTest.class);
     }