Merge branch '589-when-a-message-is-shared-share-its-transitive-dependencies' into 'master'

When a message is shared, share its transitive dependencies

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 unshared messages with shared
dependents.

Closes #589

See merge request !325