Use persistence functionality throughout all modules
As discussed in #21272 comment:18 and before.
All storing of files should be done by the persist-mechanism, i.e., a o.t.c.persist.*Persistence classes should also be used for storing descriptors initially (i.e., when fetched from authorities).
Goal: We want to identify a common approach for storing descriptors during sync-runs and imports. The various modules should diverge from it as little as possible. The resulting changes (once the approach is defined) will be implemented in the various modules and the implementation of these changes will make use of the persistence classes in order to avoid code multiplication for file storing/writing/renaming/etc. Currently, there is quite a variety of handling storage that is in many cases historically grown but not due necessity (cf. comment:1).