Fix synchronization issue

[lontivero]

Access to the banning times cache was not correctly synchronized and that resulted in exceptions like the following:

2023-10-30 02:37:10.382 [264] ERROR     PeriodicRunner.ExecuteAsync (107)       System.IndexOutOfRangeException: Index was outside the bounds of the array.
   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   at WalletWasabi.WabiSabi.Backend.DoSPrevention.Prison.GetBanTimePeriod(OutPoint outpoint, DoSConfiguration configuration)
   at WalletWasabi.WabiSabi.WabiSabiCoordinator.<>c__DisplayClass35_0.<BanDescendant>g__IsInputBanned|0(TxIn input)
   at System.Linq.Enumerable.WhereSelectListIterator`2.ToArray()
   at WalletWasabi.WabiSabi.WabiSabiCoordinator.<>c__DisplayClass35_0.<BanDescendant>b__3(Transaction tx)
   at System.Linq.Enumerable.WhereSelectListIterator`2.MoveNext()
   at System.Linq.Enumerable.WhereEnumerableIterator`1.MoveNext()
   at System.Linq.Enumerable.SelectManySingleSelectorIterator`2.MoveNext()
   at WalletWasabi.WabiSabi.WabiSabiCoordinator.BanDescendant(Object sender, Block block)
   at WalletWasabi.Blockchain.Blocks.BlockNotifier.ActionAsync(CancellationToken cancel)

Additionally here we don't ban spent inputs because they cannot participate anyway.

[turbolay]

utACK