All-day tasks should have `null` time zone (update db, contract, processors, etc)

[lemonboston]

The current handling of the time zone field for all-day tasks is not clear and may not be consistent, in order to create a clearer situation, update everything to use null for the time zone in these cases.

Things to update:

• Related TaskContract docs
• Bump DB version and add an update procedure to set timezone to null for tasks with IS_ALLDAY true
• Update the provider's EntityProcessor(s) to set the timezone to null when inserting/updating all-day tasks.
• Check FieldAdapters and remove any fault tolerance regarding this issue