Add BdfReader methods for 32-bit ints.

We use these a lot so it's convenient to have built-in support.

Also make BdfReaderImpl and BdfWriterImpl final to enable compiler optimisations.

bramble-api/src/main/java/org/briarproject/bramble/api/data/BdfReader.java

@@ -32,6 +32,12 @@ public interface BdfReader {
 
 	void skipLong() throws IOException;
 
+	boolean hasInt() throws IOException;
+
+	int readInt() throws IOException;
+
+	void skipInt() throws IOException;
+
 	boolean hasDouble() throws IOException;
 
 	double readDouble() throws IOException;