Recover from messages dropped by the network

[dplyukhin]

My thesis describes how we can use ingress/egress stages to detect when messages are dropped over the network. This has yet to be implemented.

Some TODOs off the top of my head:

• [ ] When the ingress actor receives an ingress entry, scan the entry to detect dropped messages;
• [ ] When the ingress actor detects dropped messages, notify the recipient actor;
• [ ] When the recipient is notified, it should update its local state and possibly send an update to the GC;
• [ ] Ingress/egress actors and GC actors need to implement reliable messaging logic, in case their messages to each other are dropped;
• [ ] Testing.

This issue doesn't cover messages dropped locally, e.g.

• An actor on system A sends messages to a recipient on system B faster than they can be serialized, causing messages to be dropped by the queue;
• An actor drops messages because it has a bounded mailbox.

We should be able to handle those problems too, but let's discuss that in a separate issue.