Database performance

[josephg]

The redis database layer is pretty slow at the moment because the code insists on storing / retrieving everything from the server on every operation and query.

There are two easy ways this could be made faster:

• Introduce a document cache between the database and the model code
• Make the database layer only commit new document snapshots infrequently.

This change should make the redis database perform about as well as the memory database, which is about a 2x performance boost to the whole system.

[josephg]

It turns out a caching layer is needed for document metadata anyway. Implementing this now.

[josephg]

Done: 52be4fd