Max age should be optional #440

Expected Behavior

Setting max_age to 0 (or not configuring it) should allow users to look up data regardless of age.

Current Behavior

During batch retrieval, no data is returned when max_age is set to 0.

Steps to reproduce

1. Set max_age to 0 on a feature set
2. Retrieve features using get_batch_features with Python SDK.

Possible Solution

Only need to add conditional logic to the SQL query that retrieves feature data.