storage controller: fix handing of tenants with no timelines during scheduling optimization (#7673)

## Problem

Storage controller was using a zero layer count in SecondaryProgress as
a proxy for "not initialized". However, in tenants with zero timelines
(a legitimate state), the layer count remains zero forever.

This caused https://github.com/neondatabase/neon/pull/7583 to
destabilize the storage controller scale test, which creates lots of
tenants, some of which don't get any timelines.

## Summary of changes

- Use a None mtime instead of zero layer count to determine if a
SecondaryProgress should be ignored.
- Adjust the test to use a shorter heatmap upload period to let it
proceed faster while waiting for scheduling optimizations to complete.

storage_controller/src/service.rs

@@ -4745,7 +4745,7 @@ impl Service {
                     // them in an optimization
                     const DOWNLOAD_FRESHNESS_THRESHOLD: u64 = 10 * 1024 * 1024 * 1024;
 
-                    if progress.bytes_total == 0
+                    if progress.heatmap_mtime.is_none()
                         || progress.bytes_total < DOWNLOAD_FRESHNESS_THRESHOLD
                             && progress.bytes_downloaded != progress.bytes_total
                         || progress.bytes_total - progress.bytes_downloaded