enforce key constraints on create & update

[sullivan-]

• turn on key enforcement via config
• for primary keys and mongo, this should already be handled by basic schema generation.
• for non-primary keys, mongo can enforce key constraint over a single partition with basic schema.
  • if we can know we are not sharded, then mongo enforces the key
• for all cassandra keys, we need to manage this concern ourselves.
• for non-partitioned keys and full enforcement, we need to manage this concern ourselves.
• we might be able to work it into the insert/update commands, but if not, we just have to select the key before insert/update.

[sullivan-]

on my story board here: https://www.pivotaltracker.com/story/show/107958610