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## Problem Shard splits break timeline imports. ## Summary of Changes Ensure mutual exclusion for imports and shard splits. On the shard split code path: 1. Right before shard splitting, check the database to ensure that no-import is on-going for the tenant. Exclusion is guaranteed because this validation is done while holding the exclusive tenant lock. Timeline creation (and import creation implicitly) requires a shared tenant lock. 2. When selecting a shard to split, use the in-mem state to exclude shards with an on-going import. This is opportunistic since an import might start after the check, but allows shard splits to make progres instead of continously retrying to split the same shard. On the timeline creation code path: 1. Check the in-memory splitting flag on all shards of the tenant. If any of them are splitting, error out asking the client to retry. On the happy path this is not required, due to the tenant lock set-up described above, but it covers the case where we restart with a pending shard-split. Closes https://github.com/neondatabase/neon/issues/11567