Caching for find() and findOne()

[vkarpov15]

Right now we wrap write ops, but not find() or findOne(). We should also store the results of find() and findOne(), so restarts don't need to execute those queries.

Couple of caveats:

1. find().cursor() is tricky
2. As a general case of (1), find() can return a lot of documents. We shouldn't expect that the result of find() can be stored in a single Operation document