v0.12.0

Changes:

• only time is required globally (across all device data types & upload metadata)
• in particular, timezoneOffset and conversionOffset were required and are now optional
• deviceTime is required on all device data types except cbg, where it's optional b/c of HealthKit integration
• flesh out the tests for all things globally required - for many things, there weren't any!
• timeProcessing on upload metadata now required and enforced to be one of three values: utc-bootstrapping, across-the-board-timezone, or none

Due to these changes we will be bumping _schemaVersion to 3 when deploying this.