Create SPI for Cross-Site Replication conflict resolution

A conflict happens if 2 sites try to update the same key (put/remove) concurrently.

With async Cross-Site Replication (aka IRAC) when a conflict is detected, we use the site name to pick the winning update.

Interface

interface ABC<V> { // or concrete class
  String site(); //the site name which wrote this value
  V value(); // the value. can be null if the key was removed
  Metadata metadata(); //can be null
}

interface Resolve<K, V> {
  ABC<V> resolve(K key, ABC<V> entry1, ABC<V> entry2);
}

Configuration

Set the class name to use. Can be a Class object (as shown above) or a String with the class name.
For server mode, you need to add your jar with the classes required to server/lib.
The configuration fails to validate if the conflict resolution class is set with SYNC strategy.

ConfigurationBuilder.sites().addBackup()
   .site("S")
  .strategy(BackupConfiguration.BackupStrategy.ASYNC)
  .conflictResolution(MyResover.class);