Moved classes from messaging package to sync package.

briar-api/src/org/briarproject/api/sync/RetentionAck.java

@@ -0,0 +1,16 @@
+package org.briarproject.api.sync;
+
+/** A packet acknowledging a (@link RetentionUpdate} */
+public class RetentionAck {
+
+	private final long version;
+
+	public RetentionAck(long version) {
+		this.version = version;
+	}
+
+	/** Returns the version number of the acknowledged update. */
+	public long getVersion() {
+		return version;
+	}
+}