Add priority record for choosing between redundant connections.

bramble-api/src/main/java/org/briarproject/bramble/api/sync/Priority.java

@@ -0,0 +1,23 @@
+package org.briarproject.bramble.api.sync;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * A record containing a nonce for choosing between redundant sessions.
+ */
+@Immutable
+@NotNullByDefault
+public class Priority {
+
+	private final byte[] nonce;
+
+	public Priority(byte[] nonce) {
+		this.nonce = nonce;
+	}
+
+	public byte[] getNonce() {
+		return nonce;
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/sync/RecordTypes.java

@@ -10,4 +10,5 @@ public interface RecordTypes {
 	byte OFFER = 2;
 	byte REQUEST = 3;
 	byte VERSIONS = 4;
+	byte PRIORITY = 5;
 }

bramble-api/src/main/java/org/briarproject/bramble/api/sync/SyncConstants.java

@@ -49,4 +49,10 @@ public interface SyncConstants {
 	 * simultaneously.
 	 */
 	int MAX_SUPPORTED_VERSIONS = 10;
+
+	/**
+	 * The length of the priority nonce used for choosing between redundant
+	 * connections.
+	 */
+	int PRIORITY_NONCE_BYTES = 16;
 }

bramble-api/src/main/java/org/briarproject/bramble/api/sync/SyncRecordReader.java

@@ -28,4 +28,8 @@ public interface SyncRecordReader {
 	boolean hasVersions() throws IOException;
 
 	Versions readVersions() throws IOException;
+
+	boolean hasPriority() throws IOException;
+
+	Priority readPriority() throws IOException;
 }

bramble-api/src/main/java/org/briarproject/bramble/api/sync/SyncRecordWriter.java

@@ -17,5 +17,7 @@ public interface SyncRecordWriter {
 
 	void writeVersions(Versions v) throws IOException;
 
+	void writePriority(Priority p) throws IOException;
+
 	void flush() throws IOException;
 }