Allow nulls in BdfList, BdfDictionary.

BdfList and BdfDictionary are no longer thread-safe, they require external locking. Metadata (which is the class that will be passed across API boundaries) is still thread-safe.

briar-api/src/org/briarproject/api/db/Metadata.java

@@ -3,4 +3,9 @@ package org.briarproject.api.db;
 import java.util.Hashtable;
 
 public class Metadata extends Hashtable<String, byte[]> {
+
+	/**
+	 * Special value to indicate that a key is being removed.
+	 */
+	public static final byte[] REMOVE = new byte[0];
 }