Add a cluster-backed message cache.

service/src/main/java/org/whispersystems/textsecuregcm/configuration/MessageCacheConfiguration.java

@@ -12,6 +12,11 @@ public class MessageCacheConfiguration {
   @Valid
   private RedisConfiguration redis;
 
+  @JsonProperty
+  @NotNull
+  @Valid
+  private RedisClusterConfiguration cluster;
+
   @JsonProperty
   private int persistDelayMinutes = 10;
 
@@ -19,6 +24,10 @@ public class MessageCacheConfiguration {
     return redis;
   }
 
+  public RedisClusterConfiguration getRedisClusterConfiguration() {
+    return cluster;
+  }
+
   public int getPersistDelayMinutes() {
     return persistDelayMinutes;
   }