Update hook
This merge request models the effort in !63 (merged) and introduces an update hook.
My use-case is the following:
- instead of polling a table for changes
- i register a hook and if there are changes in the table, i query them and process them
This could be a step towards general hook support (#125). As of now it is serving a subset of all hook features.
Let me know if you would like to include this change, and if I need to change something.