Storages are now able to keep some vars in memory in their own environment.
For instance, if the storage tree is revision + local, "revision" will have an
environment and "local" will have another one.
For the futur "replicate revision storage", the environment will contain objects
in order to get the good revision for the good substorage.
Of course, if there are several "replicate revision storage", every one will have
its own environment.