Change notification for queries

[losfair]

The current primitives provided by mvstore/mvsqlite is enough to implement efficient change notification for queries: notify the app when the result of running a query may change.

The mechanisms involved are read_set and the interval field provided by the /stat endpoint. Pages read by a transaction can be recorded, and later checked against /stat response.

However, exposing this would either need modification to SQLite itself, or the use of thread-local state.

[gedw99]

Cool use case