Ensure unique timestamp locally for paxos

patch by stefania; reviewed by slebresne for CASSANDRA-9649

CHANGES.txt

@@ -1,4 +1,5 @@
 2.0.17
+ * Avoid ballot clash in Paxos (CASSANDRA-9649)
  * Improve trace messages for RR (CASSANDRA-9479)
  * Fix suboptimal secondary index selection when restricted
    clustering column is also indexed (CASSANDRA-9631)

src/java/org/apache/cassandra/service/StorageProxy.java

@@ -346,10 +346,7 @@ public class StorageProxy implements StorageProxyMBean
         int contentions = 0;
         while (System.nanoTime() - start < timeout)
         {
-            long ballotMillis = summary == null
-                              ? System.currentTimeMillis()
-                              : Math.max(System.currentTimeMillis(), 1 + UUIDGen.unixTimestamp(summary.mostRecentInProgressCommit.ballot));
-            UUID ballot = UUIDGen.getTimeUUID(ballotMillis);
+            UUID ballot = UUIDGen.getTimeUUID();
 
             // prepare
             Tracing.trace("Preparing {}", ballot);