DatabaseComponent throws an exception instead of returning silently if a contact is removed during an operation involving that contact. More unit tests.

Unit tests and bugfixes for DatabaseComponent. Merged code from various unique ID classes into a common superclass.

other groups no longer affect sendability, which makes it safe to delete all
messages from a group when unsubscribing.

Changed "neighbour" to "contact" throughout (messy, but it's only going to get messier later). Also reordered some methods in DatabaseComponent impls.