iddb: rich query with cache invalidation

[dckc]

idea: add a select method that takes a set of tables paired with key ranges for those tables and a "callback" channel. Return matching rows; then, when the DB changes such that this query might return different results, fire the callback.

problem: execution of the callback is charged to the deployer that made the DB change, and it cost an arbitrary amount.

another idea: keep a state generation counter. return it with select results. support if-modified-since style