Major pgsql rework.

Gone is the ugly KORE_PGSQL macro that hid an overly complex
state machine for the pgsql api.
Gone is the pgsql array that was attached to http_requests.
Gone are the callback hacks inside the pgsql api.

Instead, I strongly encourage people to use the new state machine
api Kore offers to properly deal with asynchronous queries.

The pgsql example in examples/pgsql has been updated to reflect
these changes.