Add metadata indexing to dropbox manager

[bryan-harter]

• File indexer uses ipynotify which is no longer supported.
• ipynotify runs all the time and when something happens in the file system it triggers python in a new thread to handle it (sort of like a lambda function on the server)
• Without this, we'll need to wait a day to handle files, could cause issues when iterating during deliveries

Solution: dropbox manager can index into the database. The indexer can continue to run on some cadence to check that the file system and database are in sync

First need to investigate if adding metadata indexing to dropbox manager would have any unintended consequences

[bryan-harter]

Also, remove the delta indexer code from the code base