Use-after-free in KV backends via leaked `affinitypool::spawn_local` future

The kv-mem, kv-surrealkv and kv-rocksdb backends move non-'static borrowed state into affinity-pool closures through affinitypool::spawn_local: a tokio::sync::MutexGuard<Option<TransactionInner>>, &Transaction, &mut ScanStateKeys/Vals, &mut dyn KeyVisitor/ValVisitor, RwLockReadGuard, and a ReadOptions holding a raw rocksdb_snapshot_t*. In affinitypool 0.7.0 spawn_local is safe and erases the 'pool lifetime via async_task::Builder::spawn_unchecked; the only thing keeping it sound is the Drop implementation of the returned future, which blocks until the worker stops using the borrows.

Safe code can leak that future (mem::forget, Box::leak, ManuallyDrop, a reference cycle, or a leaked enclosing future). The borrows then end while the pool worker still uses them, producing a use-after-free on the transaction allocation; AddressSanitizer reports a read of freed mutex data and a write when the worker unlocks the freed tokio::sync::Mutex on the pool thread. No unsafe is required at the call site, and the affected methods include count, keys/keysr, scan/scanr, compact, shutdown, point reads and writes, and cursor iteration.

3.3.0 upgrades to affinitypool 0.8.0, where spawn_local is an unsafe function and the caller must uphold the leak-freedom invariant (commit 8156ff69).