DfsPackParser: Create object indices if config says so

DfsPackParser: Create object indices if config says so

The DfsInserter writes the pack and its indices in the flush() method,
but when the writing happens via DfsPackParser, it is the parser which
writes the pack and indices. When combined with a parser, flushing the
inserter is a noop.

Add the writing of the object size index to the packparser#parse
method, mirroring how the primary index is written.

Change-Id: I52c5db153fea7e4a8ecd8b3d5de7ad21f7f81a60