feat(pageserver): repartition on L0-L1 boundary (#10548)

## Problem

Reduce the read amplification when doing `repartition`.

## Summary of changes

Compute the L0-L1 boundary LSN and do repartition here.

---------

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"],