Actor reference leak in AddressTerminatedTopic

[petrikero]

In case where actor A, that is being watched by remote actor B, terminates immediately after being unwatched by B, there is a chance the AddressTerminatedTopic leaks a reference to A.

The issue arises due to TellWatchersWeDied() clearing the incorrect "watching" list when it should clear the watchedBy list in its finally block. Also, the clearing should be wrapped in a MaintainAddressTerminatedSubscription().

For reference, original Akka already implements the proposed change.

I have a pending fix for this and will make a PR.

Using Akka.Net version 1.3.17, though bug seems to have been there for years. Bug appears on Windows and dockerized Linux. I don't have a simple repro, but am seeing it in a larger system.

[petrikero]

Here's a link to the Akka implementation in tellWatchersWeDied(): https://github.com/akka/akka/blob/6bf20f4117a8c27f8bd412228424caafe76a89eb/akka-actor/src/main/scala/akka/actor/dungeon/DeathWatch.scala#L133

[petrikero]

Here's a link to the PR as well: #4305