[HER-1546] Springify(5): Update checkpointing to work smoothly with spring-configured crawls

stats & logging checkpoint support
* StatisticsTracker.java
    implement Checkpointable; save small state in JSON, recycle BDB data when recovering
* CrawlerLoggerModule.java
    implement Checkpointable; rotate logs on checkpoint
* GenerationFileHandler.java
    avoid clobbering leftover logs from earlier futures via moveAside
* CrawlStatSnapshot.java
    (sameProgressAs) test for lack-of-progress
* ObjectIdentityMemCache.java, TopNSet.java
    offer deeper access to assist checkpoint/resume

commons/src/main/java/org/archive/io/GenerationFileHandler.java

@@ -100,6 +100,7 @@ public class GenerationFileHandler extends FileHandler {
              storeSuffix;
         File activeFile = new File(filename);
         File storeFile = new File(storeFilename);
+        FileUtils.moveAsideIfExists(storeFile);
         if (!activeFile.renameTo(storeFile)) {
             throw new IOException("Unable to move " + filename + " to " +
                 storeFilename);

commons/src/main/java/org/archive/util/ObjectIdentityMemCache.java

@@ -19,6 +19,7 @@
  
 package org.archive.util;
 
+import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
 
@@ -72,4 +73,12 @@ implements ObjectIdentityCache<String,V> {
     public void sync() {
         // do nothing
     }
+
+    /**
+     * Offer raw map access for convenience of checkpoint/recovery.
+     * @return Map<String, V>
+     */
+    public Map<String, V> getMap() {
+        return map;
+    }
 }