When receiving a public activity in an inbox which is marked as "shared" for multiple actors, we should save the activity in their inboxes.
It relates to #174
The main issue is how to find all actors that share that inbox. The iterative solution is to go through all local actors and filter those that have their Endpoints.SharedInbox property set to the one we're looking for. This doesn't sound very efficient though.
I think this should work by using the Inbox collection where the activity was received, and then match that IRI to all the actors on the same instance that use it as a sharedInbox.
Modified the processing package to receive the inbox/outbox where the activity was received as a parameter to the ProcessingActivity function in 92a8ca7901