Constraint creation in HA clusters.

 o To protect from race conditions between constraints coming online on master long before they are enforced on slaves,
   master now rejects transactions that were started before a constraint was brought online.

 o This initial implementation does not verify if these started-before transactions fail the new constraint, it simply
   fails all. The reasoning here is that the window for this is a few milliesconds to a second, and it will only occur
   when an admin has manually brought a constraint online. As such, it felt uneccessary to duplicate the constraint
   validation code.
Merge pull request #1212 from jakewins/ha-constraints

Constraint creation in HA clusters.