storage controller: after shard split, some shards end up with heatmap uploads disabled.

[jcsp]

At the start of do_tenant_shard_split, we drop any secondary location for the parent shards. The reconciler uses presence of secondary locations as a condition for enabling heatmaps.

On the pageserver, child shards inherit their configuration from parents, but the storage controller assumes the child's ObservedState is the same as the parent's config from the prepare phase. The result is that some child shards end up with inaccurate ObservedState, and until something next migrates or restarts, those tenant shards aren't uploading heatmaps, so their secondary locations are downloading everything that was resident at the moment of the split (including ancestor layers which are often cleaned up shortly after the split).