DelegatingAuroraConnection: Allow more control on delegate choice

This allows you to specify more precise what type of logic should be used in picking a delegate connection.
Previously the only control was through setting the readOnly status.  This behavior is still defaulted as the `Smart` option.
But in addition behaviors to use only the master, or only a secondary, or even distribute to specific secondary servers have been added.
This can allow for more performant control over which connections to use, but can also allow options to make better use of the cluster.

Please read the javadocs at the top of `DelegatingAuroraConnection` for more details on the different behaviors this now provides.

Author: jentfoo

arcCommon/src/main/java/org/threadly/db/aurora/AuroraClusterMonitor.java

@@ -76,24 +76,59 @@ protected AuroraClusterMonitor(SubmitterScheduler scheduler, long checkIntervalM
     clusterStateChecker = new ClusterChecker(scheduler, checkIntervalMillis, driver, clusterServers);
     replicaIndex = new AtomicLong();
   }
+  
+  /**
+   * Getter for the current number of healthy replicate servers known in the cluster.  The master 
+   * server is not included in this count.
+   * 
+   * @return The number of healthy replica servers in the cluster
+   */
+  public int getHealthyReplicaCount() {
+    return clusterStateChecker.secondaryServers.size();
+  }
 
   /**
    * Return a random read only replica server.
    *
    * @return Random read only replica server, or {@code null} if no servers are healthy
    */
   public AuroraServer getRandomReadReplica() {
-    long replicaIndex = this.replicaIndex.getAndIncrement();
-    int secondaryCount;
-    // may loop if secondary server becomes unhealthy during check
-    while ((secondaryCount = clusterStateChecker.secondaryServers.size()) > 0) {
+    while (true) {
+      int secondaryCount = clusterStateChecker.secondaryServers.size();
       try {
-        return clusterStateChecker.secondaryServers.get((int)(replicaIndex % secondaryCount));
+        if (secondaryCount == 1) {
+          return clusterStateChecker.secondaryServers.get(0);
+        } else if (secondaryCount == 0) {
+          return null;
+        } else {
+          long replicaIndex = this.replicaIndex.getAndIncrement();
+          return clusterStateChecker.secondaryServers.get((int)(replicaIndex % secondaryCount));
+        }
       } catch (IndexOutOfBoundsException e) {
         // secondary server was removed during check, loop and retry
       }
     }
-    return null;
+  }
+  
+  /**
+   * Return a read replica which corresponds to an index.  A given index may not always provide the 
+   * same server depending on membership changes or other re-ordering that may occur.  But should 
+   * provide a basic way to get varying servers when the internal randomization mechanism is not 
+   * desired.
+   * 
+   * @param index Zero based index of replica to provide
+   * @return Read only replica server, or {@code null} if no servers was at the provided index
+   */
+  public AuroraServer getReadReplica(int index) {
+    if (clusterStateChecker.secondaryServers.size() <= index) {
+      return null;
+    }
+    try {
+      return clusterStateChecker.secondaryServers.get(index);
+    } catch (IndexOutOfBoundsException e) {
+      // secondary server was removed after check
+      return null;
+    }
   }
 
   /**