Squash OffsetCache into WindowCache

Originally when I wrote this code I had hoped to use OffsetCache
to also implement the UnpackedObjectCache.  But it turns out they
need rather different code, and it just wasn't worth trying to
reuse the OffsetCache base class.

Before doing any major refactoring or code cleanups here, squash the
two classes together and delete OffsetCache.  As WindowCache is our
only subclass, this is pretty simple to do.  We also get a minor
code reduction due to less duplication between the two classes,
and the JIT should be able to do a better job of optimization here
as we can define types up front rather than relying on generics
that erase back to java.lang.Object.

Change-Id: Icac8bda01260e405899efabfdd274928e98f3521
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>