Remove SoftReference from dfs.DeltaBaseCache

Remove SoftReference from dfs.DeltaBaseCache

The Java GC doesn't always clear these before running out of memory
and failing allocations. In practice OpenJDK 7 is leaving these live,
removing any advantage of the SoftReference to attempt to shed memory
when the GC is unable to continue allocating.

Instead follow the pattern of the DfsBlockCache and use hard refs
to the object data. Require applications to configure the cache
size more accurately given expected memory usage.

Change-Id: I87586b3e71b1cba0308a6a278d42e971be4bccd3