Libprocess should have a way to detect stuck actor.

We spent two days on a bug, which turns out to be an infinite loop in an actor, blocking other events from being processed by that actor.

Currently, the only way to know about a stuck actor is to use gdb. We should think about a way to print error logs when an actor has stuck for more than a threshold.

For instance, Linux kernel will print a warning in kernel log if a task is stuck for more than 120 seconds. Something like this will be extremely helpful.

Another way is to expose some metrics around this.