new pagination interface, more stable reads, Bfabric.from_config

[leoschwarz]

This mixes 3 different concerns, but it will make it much quicker for me to bundle these changes than separate them out from the branch where they are integrated already:

• New pagination logic:
  • Specify max number of items
  • Specify offset of items to skip
• Stable reading
  • Reading passes a createdbefore query field (if it's not part of the query) to ensure consistent reads/pagination in the presence of insertions into the database. Deletions are not handled and might require an API feature.
  • It will need to be tested further before releasing, but my idea is that this is a lot more flexible moving forward
• New Bfabric.from_config arguably it is a bit redundant with the get_system_auth method, however I do feel like 99% of use cases will be handled with this method now and it will be easier to do so. I'm not changing the usage in this PR yet.