Fix full state updates (#4833)

* Clear entities seen by a session when the full update is requested

* Disable PVS budged for the full update

It helps preventing weird intermediate states when a client observes
themselves in a void with all the alert notifications going wild.
However it introduces a spike on CPU and networking.

* Add changelog line

* Move the changelog line to the proper place

* Update RELEASE-NOTES.md

* Revert "Disable PVS budged for the full update"

This reverts commit 6976ca04b8.

* Update RELEASE-NOTES.md

---------

Co-authored-by: metalgearsloth <31366439+metalgearsloth@users.noreply.github.com>

RELEASE-NOTES.md

@@ -43,7 +43,7 @@ END TEMPLATE-->
 
 ### Bugfixes
 
-*None yet*
+* Fix broken full state updates.
 
 ### Other
 

Robust.Server/GameStates/PvsSystem.Session.cs

@@ -193,5 +193,6 @@ internal sealed partial class PvsSystem
 
         session.PreviouslySent.Clear();
         session.LastSent = null;
+        session.Entities.Clear();
     }
 }