Allow object reuse selection to occur in parallel

Allow object reuse selection to occur in parallel

ObjectReader implementations may wish to use multiple threads in
order to evaluate object reuse faster.  Let the reader make that
decision by passing the iteration down into the reader.

Because the work is pushed into the reader, it may need to locate a
given ObjectToPack given its ObjectId.  This can easily occur if the
reader has sent a list of ObjectIds to the object database and gets
back information keyed only by ObjectId, without the ObjectToPack
handle.  Expose lookup using the PackWriter's own internal map,
so the reader doesn't need to build a redundant copy to track the
assocation of ObjectId back to ObjectToPack.

Change-Id: I0c536405a55034881fb5db92a2d2a99534faed34
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>