reserve time and seq_num keys

[gwbischof]

user-defined data_keys called "time" or "seq_num" conflict with with bluesky generated keys of the same name. This PR reserves this keys, preventing users from using them.

[gwbischof]

This is a useful stackoverflow post about using not in jsonschema: https://stackoverflow.com/questions/61059683/how-to-conditionally-forbid-properties-based-on-presence-of-other-properties-in/61062869#61062869

[tacaswell]

Do we have any users currently using them?

[dmgav]

@tacaswell https://github.com/NSLS-II-SRX/profile_collection/blob/06b6d22ba27f75ff1b775c794f68889a02278be3/startup/32-zebra.py#L847

[danielballan]

The intent is to pair this with a migration tool (similar to "shape fixer") to alter the names on existing data.

[danielballan]

This needs:

• a substantial rebase, as the system for encoding schemas has evolved substantially
• a clear story for how to deal with existing usage