Use CRUD-names for methods in ConfigurationManager

Also reorder methods according to the acronym and raise
NotImplementedError in them.
Add ConfigurationManagerMongoDB stub
Add json_decoder/encoder attribute to `Message'

Both the `ConfigurationManager' and the `Context' get parameters to
set the json_decoder and json_encoder.
Add `reply_error' to `Context'
Implement `handle_request*' methods
Remove main procedure from ConfigurationManager