MemoryPool: consider signers intersection on Conflicts check

This patch should be a part of fd1748d, but
was accidentally skipped. This patch matches exactly the NeoGo behaviour that
was added in nspcc-dev/neo-go#3061 and that was
discussed earlier in neo-project#2818 (comment).

Fix the issue described in neo-project#2818 (comment).

Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>

src/Neo/Ledger/MemoryPool.cs

@@ -369,7 +369,7 @@ private bool CheckConflicts(Transaction tx, out List<PoolItem> conflictsList)
             {
                 if (_unsortedTransactions.TryGetValue(hash, out PoolItem unsortedTx))
                 {
-                    if (!tx.Signers.Select(p => p.Account).Contains(unsortedTx.Tx.Sender)) return false;
+                    if (!tx.Signers.Select(p => p.Account).Intersect(unsortedTx.Tx.Signers.Select(p => p.Account)).Any()) return false;
                     conflictsFeeSum += unsortedTx.Tx.NetworkFee;
                     conflictsList.Add(unsortedTx);
                 }