No redeploy

This fixes #148.

* Moves validation stuff out of `db.clj` into a specific validation function in `event.clj`.
* `update-jar` is removed.
* `add-jar`'s `check-only` arity is removed.
* HTTP deploys only write to the DB when the pom is deployed. Previously the jar would write if no row existed, and then the record would get updated by the pom's deploy.

The last point I think we could back out if there are objections. Enforcing immutability on a row level in the DB isn't that important if we're hoping to move to the event stream model soon anyway.

Thoughts?