Simplify a bit the multiplexer and readonly filter objstorages and other refactorings
This MR consists in 4 revisions (and is based on !149 (merged)):
- Convert the ObjStorageFilter as a simple proxy ObjStorage
- Simplify a bit the MultiplexerObjStorage factory
- Rename ReadOnlyObjStorage and NonIterableObjStorage exceptions
- Make the ReadOnlyObjStorageProxy raise ReadOnlyObjStorageError when needed
Edited by David Douard