Added the 'delta2' compressor, which is very similar to the 'delta'
compressor, except that it should be easier to understand.
With another modification, it should also be as efficient on the wire.
Instead of using a tree of keys containing trees of values and a
linked-list of LRU entries, it uses a straight-up LRU of key-value entries.

Also, the various codecs were modified to include the .name field as now
required.