Fix memory leak in dfs.DeltaBaseCase

Fix memory leak in dfs.DeltaBaseCase

The LRU chain management code was broken leading to situations where
the chain was incomplete.  This prevented the cache from removing
items when it exceeded its memory target, causing a leak.

One case was repeated hit on the head of the chain. moveToHead(e)
was invoked linking the head back to itself in a cycle orphaning
the rest of the table.

Add some unit tests to cover this and a few other paths.

Change-Id: Ib27486eaa1b1d2bf1c745a56d0a5832bfb029322