Added serializable to preemptive break out of retry loop.

Author: twilmes

src/main/java/com/experoinc/janusgraph/diskstorage/foundationdb/FoundationDBTx.java

@@ -118,7 +118,8 @@ public synchronized void commit() throws BackendException {
                 failing = false;
                 break;
             } catch (IllegalStateException | ExecutionException e) {
-                if (isolationLevel.equals(IsolationLevel.READ_COMMITTED_NO_WRITE)) {
+                if (isolationLevel.equals(IsolationLevel.SERIALIZABLE) ||
+                        isolationLevel.equals(IsolationLevel.READ_COMMITTED_NO_WRITE)) {
                     break;
                 }
                 restart();