interface and a factory for that.

added a cache to the IdentityManager, changed its signature, modified when and where the author is stored

made the author creation single-threaded again in the LifecycleManager, removed redundant code

in group metadata to be able to speed up group listings.

Closes #584, #586, #585

Like other recursive operations on the dependency graph, this is
not done in a single transaction to prevent an attacker from creating
arbitrary large transactions.

So at startup, the `ValidationManager` finds and resumes any
unfinished operations, by looking for shared messages with unshared
dependencies.

This also adds unit tests to prevent regressions like this in the
future.

The docs say this can happen for a directory if there's an I/O error.