support SQL-standard date/datetime functions

This allows setting date and datetime type fields to the values `CURRENT_DATE`, `CURRENT_TIMESTAMP`, and `LOCALTIMESTAMP` from the API, and translates that into SQL expressions that get evaluated by the database.

This allows client-side code to set a date or timestamp to current as seen by the server, in case the client computer's clock is incorrect.
Revert "Adds a wrapper around abort function to add JSON Content-Type."

This reverts commit 8aacd39d91777f10153424e47cbb5d1c2833491a.

Conflicts:
	tests/__init__.py
	tests/test_exceptions.py
allow disabling deep results per-request

originally by @johaness
allow fine grained control of relations included in output
bump to 1.current-upstream-version