Support compaction_threshold=1 for tiered compaction (#7257)

Many tests like `test_live_migration` or
`test_timeline_deletion_with_files_stuck_in_upload_queue` set
`compaction_threshold` to 1, to create a lot of changes/updates. The
compaction threshold was passed as `fanout` parameter to the
tiered_compaction function, which didn't support values of 1 however.
Now we change the assert to support it, while still retaining the
exponential nature of the increase in range in terms of lsn that a layer
is responsible for.

A large chunk of the failures in #6964 was due to hitting this issue
that we now resolved.

Part of #6768.

pageserver/compaction/src/compact_tiered.rs

@@ -43,7 +43,8 @@ pub async fn compact_tiered<E: CompactionJobExecutor>(
     fanout: u64,
     ctx: &E::RequestContext,
 ) -> anyhow::Result<()> {
-    assert!(fanout >= 2);
+    assert!(fanout >= 1, "fanout needs to be at least 1 but is {fanout}");
+    let exp_base = fanout.max(2);
     // Start at L0
     let mut current_level_no = 0;
     let mut current_level_target_height = target_file_size;
@@ -106,7 +107,7 @@ pub async fn compact_tiered<E: CompactionJobExecutor>(
             break;
         }
         current_level_no += 1;
-        current_level_target_height = current_level_target_height.saturating_mul(fanout);
+        current_level_target_height = current_level_target_height.saturating_mul(exp_base);
     }
     Ok(())
 }