Summary:
- Change RECORD's dates_as_str default to False.
- Reimplement objtype encode_object/decode_object with less machinery.
- Implement encoding of dicts (with string keys).
- Make lists and dicts encode values recursively.
- Implement encoding/decoding in the client
- Decode automatically in plugins' fetchSelectedTable/Record, with an option to skip.
Test Plan: Tested manually, not sure what tests may be affected yet.
Reviewers: paulfitz
Reviewed By: paulfitz
Differential Revision: https://phab.getgrist.com/D2593
