Add versions record to sync protocol.

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

@@ -9,5 +9,5 @@ public interface RecordTypes {
 	byte MESSAGE = 1;
 	byte OFFER = 2;
 	byte REQUEST = 3;
-
+	byte VERSIONS = 4;
 }

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

@@ -35,4 +35,10 @@ public interface SyncConstants {
 	 * The maximum number of message IDs in an ack, offer or request record.
 	 */
 	int MAX_MESSAGE_IDS = MAX_RECORD_PAYLOAD_BYTES / UniqueId.LENGTH;
+
+	/**
+	 * The maximum number of versions of the sync protocol a peer may support
+	 * simultaneously.
+	 */
+	int MAX_SUPPORTED_VERSIONS = 10;
 }

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

@@ -25,4 +25,7 @@ public interface SyncRecordReader {
 
 	Request readRequest() throws IOException;
 
+	boolean hasVersions() throws IOException;
+
+	Versions readVersions() throws IOException;
 }

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

@@ -15,5 +15,7 @@ public interface SyncRecordWriter {
 
 	void writeRequest(Request r) throws IOException;
 
+	void writeVersions(Versions v) throws IOException;
+
 	void flush() throws IOException;
 }

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

@@ -0,0 +1,26 @@
+package org.briarproject.bramble.api.sync;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.util.List;
+
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * A record telling the recipient which versions of the sync protocol the
+ * sender supports.
+ */
+@Immutable
+@NotNullByDefault
+public class Versions {
+
+	private final List<Byte> supported;
+
+	public Versions(List<Byte> supported) {
+		this.supported = supported;
+	}
+
+	public List<Byte> getSupportedVersions() {
+		return supported;
+	}
+}