Make DeltaBaseCache per-ObjectReader

Make DeltaBaseCache per-ObjectReader

The 'Counting objects' phase of PackWriter requires good hit rates
from the DeltaBaseCache while walking trees, the deltas need to find
their bases in the cache in order to inflate in a reasonable time.

If JGit is running in a multi-threaded server, such as Gerrit Code
Review, each thread needs its own DeltaBaseCache to prevent one thread
from evicting the other thread's relevant bases.  Move the cache to be
per-ObjectReader, lazily allocated when required by a PackFile.

Change-Id: If9d5ed06728e813632ae96dcfb811f4860b276e8
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>