Improve API key checking

[pezholio]

We removed the database level checking, as it was causing too much latency (see https://github.com/OpenAddressesUK/ernest/blob/master/lib/ernest.rb#L41), but we should really put this back, or figure out a better way of doing things.

This may be moot however, if we decide that the only way of adding things is via Turbot, in which case, we can remove the write part of the API completely.