Allow multiple implementations of logging.ContextFilter

[benhalstead]

Allow multiple components to implement logging.ContextFilter. These source components will be wrapped into a single implementation logging.MergedContextFilter which reads from the source components.

Control over the which source's version of a key/value is used in the merged view can be controlled by implementing the logging.PrioritisedContextFilter interface to return an int64 'score' where a higher number is a higher priority. A context filter not implemented that interface is assumed to have a priority of zero.