Juggle imports to avoid circularity better.
Use MessageMetadataHelper to get and cache tag info and tagpool metadata in middleware and routing.
Merge branch 'develop' into feature/issue-819-more-mdh-caching
Tests for the new tag-related methods on MessageMetadataHelper.
Merge branch 'feature/issue-819-more-mdh-caching' into develop
Merge branch 'develop' into feature/issue-818-kill-DebitAccountMiddleware

Conflicts:
	go/vumitools/middleware.py
Kill CreditManager.