Make Uuids use default random

Before this commit `Uuids` used to initiate `Random `at the point when it needs
a random number.
Not only it is not effecient it also produced bad distribution which
resulted in `should_generate_unique_random_uuids_Random` to fail, due to
inability of `Uuids::random `generate unique vales.

Author: dkropachev

Diff for: core/src/main/java/com/datastax/oss/driver/api/core/uuid/Uuids.java

@@ -75,6 +75,8 @@
  */
 public final class Uuids {
 
+  private static final SplittableRandom defaultRandom = new SplittableRandom();
+
   /** The system property to use to force the value of the process ID ({@value}). */
   public static final String PID_SYSTEM_PROPERTY = "com.datastax.oss.driver.PID";
 
@@ -232,7 +234,7 @@ private static String getProcessPiece() {
       }
     }
     if (pid == null) {
-      pid = new Random().nextInt();
+      pid = defaultRandom.nextInt();
       LOG.warn("Could not determine PID, falling back to a random integer: {}", pid);
     }
     ClassLoader loader = Uuids.class.getClassLoader();
@@ -281,7 +283,7 @@ private static long makeClockSeqAndNode() {
    */
   @NonNull
   public static UUID random() {
-    return random(new Random());
+    return random(defaultRandom);
   }
 
   /**