feat(pageserver): create image layers at L0-L1 boundary (#12023)

## Problem

Previous attempt https://github.com/neondatabase/neon/pull/10548 caused
some issues in staging and we reverted it. This is a re-attempt to
address https://github.com/neondatabase/neon/issues/11063.

Currently we create image layers at latest record LSN. We would create
"future image layers" (i.e., image layers with LSN larger than disk
consistent LSN) that need special handling at startup. We also waste a
lot of read operations to reconstruct from L0 layers while we could have
compacted all of the L0 layers and operate on a flat level of historic
layers.

## Summary of changes

* Run repartition at L0-L1 boundary.
* Roll out with feature flags.
* Piggyback a change that downgrades "image layer creating below
gc_cutoff" to debug level.

---------

Signed-off-by: Alex Chi Z <chi@neon.tech>

test_runner/regress/test_layers_from_future.py

@@ -20,6 +20,9 @@ from fixtures.remote_storage import LocalFsStorage, RemoteStorageKind
 from fixtures.utils import query_scalar, wait_until
 
 
+@pytest.mark.skip(
+    reason="We won't create future layers any more after https://github.com/neondatabase/neon/pull/10548"
+)
 @pytest.mark.parametrize(
     "attach_mode",
     ["default_generation", "same_generation"],