Add placeholder BlockSource implementation to DB.

bramble-api/src/main/java/org/briarproject/bramble/api/db/DatabaseComponent.java

@@ -208,6 +208,24 @@ public interface DatabaseComponent {
 	Collection<Message> generateRequestedBatch(Transaction txn, ContactId c,
 			int maxLength, int maxLatency) throws DbException;
 
+	/**
+	 * Returns the number of blocks in the given message.
+	 * <p>
+	 * Read-only.
+	 */
+	int getBlockCount(Transaction txn, MessageId m) throws DbException;
+
+	/**
+	 * Returns the given block of the given message.
+	 * <p>
+	 * Read-only.
+	 *
+	 * @throws NoSuchBlockException if 'blockNumber' is greater than or equal
+	 * to the number of blocks in the message
+	 */
+	byte[] getBlock(Transaction txn, MessageId m, int blockNumber)
+			throws DbException;
+
 	/**
 	 * Returns the contact with the given ID.
 	 * <p/>

bramble-api/src/main/java/org/briarproject/bramble/api/db/NoSuchBlockException.java

@@ -0,0 +1,9 @@
+package org.briarproject.bramble.api.db;
+
+/**
+ * Thrown when a database operation is attempted for a block that is not in
+ * the database. This exception may occur due to concurrent updates and does
+ * not indicate a database error.
+ */
+public class NoSuchBlockException extends DbException {
+}