Guard against multiple snapshots inserted during PUT for given doc

[nickbrowne]

This change is temporary to give us an opportunity to clean up the data and fix the db constraints.

To test:

1. Start postgres demo server: ./bin/demoserver-pg
2. Create a new document:

curl -H 'Content-Type: application/json' -X PUT -d '{ "type": "text" }' http://localhost:8000/doc/testdoc

3. Add some operations:

curl -H 'Content-Type: application/json' -X POST -d '[{"p": 0, "i": "ing"}]' http://localhost:8000/doc/testdoc?v=0
curl -H 'Content-Type: application/json' -X POST -d '[{"p": 0, "i": "test"}]' http://localhost:8000/doc/testdoc?v=1

4. Attempt to create the document again:

curl -H 'Content-Type: application/json' -X PUT -d '{ "type": "text" }' http://localhost:8000/doc/testdoc

The expected behaviour is now that step 4 returns a 404 rather than inserting an empty snapshot at version 0.
You can verify with:

SELECT * FROM snapshots WHERE doc='testdoc';

On current master an empty snapshot is erroneously created when one already exists.