rewrite_driver_factory: move instantiation up front

We need to have the NgxRewriteDriverFactory available at configuration parse
time to set options on it. This change moves instantiation of the factory in
front of configuration parsing.  Afterwards, a check is done to see if we where
enabled anywhere.  If we where not enabled / do not have any active server
contexts, the NgxRewriteDriverFactory is deleted. It's not very elegant, but we
can't think of anything better currently.

Squash merge of Otto's #165.