pageserver: enable compaction_l0_first by default (#11212)

## Problem

`compaction_l0_first` has already been enabled in production for a
couple of weeks.

## Summary of changes

Enable `compaction_l0_first` by default.

Also set `CompactFlags::NoYield` in `timeline_checkpoint_handler`, to
ensure explicitly requested compaction runs to completion. This endpoint
is mainly used in tests, and caused some flakiness where tests expected
compaction to complete.

pageserver/src/http/routes.rs

@@ -2392,6 +2392,7 @@ async fn timeline_checkpoint_handler(
     let state = get_state(&request);
 
     let mut flags = EnumSet::empty();
+    flags |= CompactFlags::NoYield; // run compaction to completion
     if Some(true) == parse_query_param::<_, bool>(&request, "force_l0_compaction")? {
         flags |= CompactFlags::ForceL0Compaction;
     }