Only remove active peer on connection failure (#79557)

Today if the `PeerFinder` experiences a connection failure then it
removes the `Peer` with the corresponding transport address. However the
removed `Peer` might be a different instance which is actually fine and
holds a connection reference which therefore leaks. With this commit we
only remove ourselves from the tracked set of peers. Relates #77295
Closes #79550

Author: DaveCTurner

server/src/main/java/org/elasticsearch/discovery/PeerFinder.java

@@ -431,7 +431,9 @@ public void onFailure(Exception e) {
                     synchronized (mutex) {
                         assert probeConnectionResult.get() == null
                             : "discoveryNode unexpectedly already set to " + probeConnectionResult.get();
-                        peersByAddress.remove(transportAddress);
+                        if (isActive()) {
+                            peersByAddress.remove(transportAddress);
+                        } // else this Peer has been superseded by a different instance which should be left in place
                     }
                 }
             });