pageserver: do all L0 compaction before image compaction (#10744)

## Problem

Image compaction can starve out L0 compaction if a tenant has several
timelines with L0 debt.

Touches #10694.
Requires #10740.

## Summary of changes

* Add an initial L0 compaction pass, in order of L0 count.
* Add a tenant option `compaction_l0_first` to control the L0 pass
(disabled by default).
* Add `CompactFlags::OnlyL0Compaction` to run an L0-only compaction
pass.
* Clean up the compaction iteration logic.

A later PR will use separate semaphores for the L0 and image compaction
passes to avoid cross-tenant L0 starvation. That PR will also make image
compaction yield if _any_ of the tenant's timelines have pending L0
compaction to further avoid starvation.

test_runner/regress/test_attach_tenant_config.py

@@ -140,9 +140,10 @@ def test_fully_custom_config(positive_env: NeonEnv):
         "compaction_period": "1h",
         "compaction_threshold": 13,
         "compaction_upper_limit": 100,
+        "compaction_l0_first": False,
         "l0_flush_delay_threshold": 25,
         "l0_flush_stall_threshold": 42,
-        "l0_flush_wait_upload": True,
+        "l0_flush_wait_upload": False,
         "compaction_target_size": 1048576,
         "checkpoint_distance": 10000,
         "checkpoint_timeout": "13m",