journal: introduce JournalStorage and JournalStorageSpace structures

This structure keeps track of specificities for a given journal type
(persistent or volatile) such as metrics, name, etc...

The cached space values are now moved in this structure so that each
journal has its own set of cached values.

Previously only one set existed and we didn't know if the cached
values were for the runtime journal or the persistent one.

When doing:

   determine_space_for(s, runtime_metrics, ...);
   determine_space_for(s, system_metrics, ...);

the second call returned the cached values for the runtime metrics.

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