Separately index time fields

[jisantuc]

Describe the bug
Time should get special treatment, since it's part of the spatio-temporal asset catalog name and items must have time specified somehow. It's also really helpful for making other queries faster. Separately indexing time will be a good first step for setting up #616

Expected behavior
Time queries don't have to use the jsonb filters -- they get special treatment using a datetime / start_datetime / end_datetime field.