Remove obsolete domain dependency

[rg2011]

Fixes issue telefonicaid/fiware-pep-steelskin#498

domain is only used to improve logging by storing information related to the request in a per-request domain. The callback provided to function getContext of logops package queries that domain to retrieve request information.

This PR replaces the global logger with a per-request instance, stored in req.logger. This instance is created by a middleware that runs early in the chain, and populated with the same information that used to be in the context.

All the middleware and handlers are also refactored to use the per-request logger instead of the global one, so we can dispose of the logger.getContext callback and the domain dependency.