Keep fallback retries single-flight and recover imagery and terrain independently

An activation that re-showed a fallback while a retry was in flight armed a
second timer, so two retries could run at once and land out of order, leaving
the cache and the screen on different providers. Retries now run one at a
time; a request made meanwhile re-arms once the current one ends, and a
result that is obsolete or no longer wanted is disposed.

Imagery and terrain retries are now handled as each settles, so a stalled
terrain attempt no longer keeps recovered imagery off screen, or the reverse.

changelog.d/1111-map-fallback-retry.md

@@ -0,0 +1,5 @@
+- A failed Esri Satellite or keyless terrain load still shows OSM or flat terrain
+  at once, but no longer lasts the whole session: while the fallback is on
+  screen, the real provider is retried in the background (30 seconds, doubling
+  to a 10-minute cap) and replaces it live once it loads. Map switches never
+  wait for a retry (Aditya, @Rohilalala, #1111).