pageserver: fix sharding emitting empty image layers during compaction (#6776)

## Problem

Sharded tenants would sometimes try to write empty image layers during
compaction: this was more noticeable on larger databases.
- https://github.com/neondatabase/neon/issues/6755

**Note to reviewers: the last commit is a refactor that de-intents a
whole block, I recommend reviewing the earlier commits one by one to see
the real changes**

## Summary of changes

- Fix a case where when we drop a key during compaction, we might fail
to write out keys (this was broken when vectored get was added)
- If an image layer is empty, then do not try and write it out, but
leave `start` where it is so that if the subsequent key range meets
criteria for writing an image layer, we will extend its key range to
cover the empty area.
- Add a compaction test that configures small layers and compaction
thresholds, and asserts that we really successfully did image layer
generation. This fails before the fix.

compute_tools/src/config.rs

@@ -51,6 +51,9 @@ pub fn write_postgres_conf(
     if let Some(s) = &spec.pageserver_connstring {
         writeln!(file, "neon.pageserver_connstring={}", escape_conf_value(s))?;
     }
+    if let Some(stripe_size) = spec.shard_stripe_size {
+        writeln!(file, "neon.stripe_size={stripe_size}")?;
+    }
     if !spec.safekeeper_connstrings.is_empty() {
         writeln!(
             file,