Open a stream for writing a pack file to the ODB.
If the ODB layer understands pack files, then the given packfile will likely be streamed directly to disk (and a corresponding index created). If the ODB layer does not understand pack files, the objects will be stored in whatever format the ODB layer uses.
function to call with progress information. Be aware that this is called inline with network and indexing operations, so performance may be affected.
payload for the progress callback