Update new_max_bookmark, Add unittests

[dsprayberry]

Description of change

• Updates new_max_bookmark to check that we do not write future-dated bookmark values (as we've observed Closeio returning future-dated activities records even when passing the date_created__lt parameter on requests)
• Adds unittests

Manual QA steps

• added 2 unittests that validate new_max_bookmark functionality::
  • test_new_max_bookmark_valid_potential
  • test_new_max_bookmark_future_dated_potential

Risks

• Minimal. State is stored in the same format, but it will no longer have the potential to be a datetime string greater than now().

Rollback steps

• revert this branch

[luandy64]

One small change, but then this is good