Send enrollment dates in ET, and align conversion windows with ET days.

[sammacbeth]

Reviewer:

Description:

Our A/B/N framework assumes that:

1. Enrollment dates are sent in ET (UTC-5).
2. Conversation windows are calculated over calendar days (in ET), rather than rolling 24-hour windows from enrollment.

Our initial implementation used UTC dates (default for javascript Date objects), and using rolling windows, so this PR corrects that by:

1. Converting the stored enrollment timestamps to a date in ET when sending.
2. Setting the enrollment date to midnight in ET when joining an experiment. This means that conversion windows will get day-aligned without us having to change the rolling-window implementation.

This also contains a fix for a potential race with the app_use metric. If it is triggered too early, we may not yet have the config processed and ready to handle metrics.

Steps to test this PR:

Automated tests:

• Unit tests
• Integration tests

Reviewer Checklist:

• Ensure the PR solves the problem
• Review every line of code
• Ensure the PR does no harm by testing the changes thoroughly
• Get help if you're uncomfortable with any of the above!
• Determine if there are any quick wins that improve the implementation

PR Author Checklist:

• Get advice or leverage existing code
• Agree on technical approach with reviewer (if the changes are nuanced)
• Ensure that there is a testing strategy (and documented non-automated tests)
• Ensure there is a documented monitoring strategy (if necessary)
• Consider systems implications

[kzar]

Mostly looks good, but just want to check about daylight savings.

[kzar]

Do we need to worry about devices that are already enrolled as well?

[sammacbeth]

Do we need to worry about devices that are already enrolled as well?

No, before we release this we'll shut down the current experiment, then start a new one, so no need to fix existing enrollments.