storcon: validate intent state before applying optimization (#12593)

## Problem

In the gap between picking an optimization and applying it, something
might insert a change to the intent state that makes it incompatible.
If the change is done via the `schedule()` method, we are covered by the
increased sequence number, but otherwise we can panic if we violate the
intent state invariants.

## Summary of Changes

Validate the optimization right before applying it. Since we hold the
service lock at that point, nothing else can sneak in.

Closes LKB-65

test_runner/performance/test_sharding_autosplit.py

@@ -73,6 +73,11 @@ def test_sharding_autosplit(neon_env_builder: NeonEnvBuilder, pg_bin: PgBin):
             ".*Local notification hook failed.*",
             ".*Marking shard.*for notification retry.*",
             ".*Failed to notify compute.*",
+            # As an optimization, the storage controller kicks the downloads on the secondary
+            # after the shard split. However, secondaries are created async, so it's possible
+            # that the intent state was modified, but the actual secondary hasn't been created,
+            # which results in an error.
+            ".*Error calling secondary download after shard split.*",
         ]
     )