perf(portals): fast-fail requests to portal hosts that stopped answering (#1421)

.changes/portals-dead-host-fast-fail.md

@@ -0,0 +1,10 @@
+---
+type: perf
+area: portals
+---
+
+Unreachable Xtream and Stalker portals no longer cost a 30-second wait per
+request. After a host fails to answer twice, further requests to it fail
+immediately for 30 seconds, so browsing a dead portal stops filling the screen
+with long spinners. Retrying, testing the connection, or editing the portal
+address contacts it again right away.