Allow multiple items to be parsed at once

[helrond]

Is your feature request related to a problem? Please describe.

We have been experiencing intermittent but persistent issues when users try to load the MyList page with many items saved. This is because an individual parse request is sent for each item.

Describe the solution you'd like

Allow Request Broker to parse multiple items at once. This will require either introducing a new method or refactoring the existing parse_item method. Changes have already been made to the get_data method to process chunks of URIs.

Additional context

Changes should be made in a way that supports updating DIMES and the Request Broker independently of one another.
Additional changes will have to be made to DIMES to support sending and handling multiple items in the parse request. See RockefellerArchiveCenter/dimes#613