Add a contactExists() method to the contactManager

This requires exposing the `containsContact()` method to the `DatabaseComponent`
and is needed for finding out efficiently whether a contact already exists.

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

@@ -162,6 +162,13 @@ public interface DatabaseComponent {
 	Collection<ContactId> getContacts(Transaction txn, AuthorId a)
 			throws DbException;
 
+	/**
+	 * Returns true if the database contains the given contact for the given
+	 * local pseudonym.
+	 */
+	boolean containsContact(Transaction txn, AuthorId remote, AuthorId local)
+			throws DbException;
+
 	/**
 	 * Returns the unique ID for this device.
 	 * <p/>