s/HTTP_STATUS_RETRY/HTTP_STATE_RETRY
Add a new "simple query" layer to our pgsql api.

This simple query allows you to ditch rolling your own
state machine for handling async pgsql states and instead
asks you to provide 3 functions:
	- init
	- results
	- done

You can see the different in complexity in the pgsql example,
which now contains a pgsql_simple.c holding the same asynchronous
query as in pgsql.c but using the simple pgsql api.

You can of course still roll your own in case you want more control.