Make BagGenerator parallelism configurable

[qqmyers]

Overview of the Feature Request
The generation of archival Bags currently uses 8 threads to read datafiles when generating the zipped Bag. As part of work for TDL related to #7493, the associated PR lowers the default to 2 threads and makes threading configurable via a :BagGeneratorThreads setting which can be used with the existing archiver classes.

What kind of user is the feature intended for?
Sysadmin

What inspired the request?
Interest in lowering the server load when archiving, particularly in the context of batch archiving as in #7493

Any related open or closed issues to this feature request? #7493