Processor: pass context in and use it.

The processor used to create a new ConfigWriter when it was initialized.
We now have one in the context, so we'll just recycle it. First of all
we'll pass the context in when creating a new Processor object, then
we'll just use context.cfg, which is granted to be initialized, wherever
we used self.cfg, and stop checking whether the config is already
initialized or not.

In the future the Processor will be able to use the context also to
acquire the BGL for LUs that require it, and to push the context down to
LUs that don't in order for them to manage their own locking.

Reviewed-by: iustinp