This diff contains the following fixes:

* Once the OOC is at OFFLAODING state, it may offload data to disk indefinitely. We should call GC manually and get OOC out of OFFLOADING if anything has changed in computation/communication pattern.
Fixes a "if" statement which entirely disabled regression for memory estimation.
* Interleaving of in resetting and calculating memory estimation potentially can cause data race.
* The superstep count in memory estimator was not coherent throughout the calculation.
* Sometime the memory estimator's accuracy is not good. We should fall back to a threshold-based scheme relying on a more pessimistic memory usage report (such as the one given by JVM – memory usage includes garbage data too, but it can be used as a pessimistic estimate, as it is currently used in ThresholdBasedOracle).

More tuning is needed for a smooth memory estimation mechanism.
"mvn clean install" passes all tests and checks.

giraph-core/src/main/java/org/apache/giraph/ooc/OutOfCoreEngine.java

@@ -426,11 +426,14 @@ public void ioCommandCompleted(IOCommand command) {
    * threads.
    *
    * @param fraction the fraction of processing threads to remain active. This
-   *                 number is in range [0, 1]
+   *                 number is in range (0, 1]
    */
   public void updateActiveThreadsFraction(double fraction) {
     checkState(fraction >= 0 && fraction <= 1);
-    int numActiveThreads = (int) (numProcessingThreads * fraction);
+    // Making sure the number of active threads is not set to 0 to ensure
+    // progress in computation
+    int numActiveThreads = Math.max(
+      (int) (Math.ceil(numProcessingThreads * fraction)), 1);
     if (LOG.isInfoEnabled()) {
       LOG.info("updateActiveThreadsFraction: updating the number of active " +
           "threads to " + numActiveThreads);
@@ -489,7 +492,7 @@ public void processingThreadFinish() {
    */
   public void updateRequestsCreditFraction(double fraction) {
     checkState(fraction >= 0 && fraction <= 1);
-    short newCredit = (short) (maxRequestsCredit * fraction);
+    short newCredit = (short) (Math.ceil(maxRequestsCredit * fraction));
     if (LOG.isInfoEnabled()) {
       LOG.info("updateRequestsCreditFraction: updating the credit to " +
           newCredit);