Spike: revisit birthdate handling

[dogversioning]

Since we are using a delta between birthdate and encounter date to calculate age at visit, we are introducing some amount of error when the ETL truncates birthdate for PHI reasons.

The spirit of this ticket is to review this decision. If we decide to keep it, we should adjust our calculation of age to be on year only and make a comment about reason. If we decide to change it, we may need an issue on the ETL side.