Don't close ObjectDatabase after parsing pack

Don't close ObjectDatabase after parsing pack

The cached object databases should not require a close to release
their cached resources. Most object databases just return their
own reference for newCachedDatabase(), so a close() here kills
the real database's internal caches, and possibly underlying files,
resulting in poor performance for the callers of PackParser like
ReceivePack or FetchProcess trying to then go look up objects that
were just parsed, or that current references point to.

Change-Id: Ia4a239093866e5b9faf82744f729fb73f4373f1a
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>