PackWriter: shed memory while creating bitmaps

PackWriter: shed memory while creating bitmaps

Once bitmap creation begins the internal maps required for packing are
no longer necessary.  On a repository with 5.2M objects this can save
more than 438 MiB of memory by allowing the ObjectToPack instances to
get garbage collected away.

Downside is the PackWriter cannot be used for any further opertions
except to write the bitmap index.  This is an acceptable trade-off as
in practice nobody uses the PackWriter after the bitmaps are built.

Change-Id: Ibfaf84b22fa0590896a398ff659a91fcf03d7128