Ensure coordinator node and cross validation nodes are different

[Neylix]

Describe the problem you discovered

When a replication node verify the validation stamps and cross validation stamps signature, it does not ensure the coordinator node and the cross validation nodes are different. This can be a breach for a malicious node to send a self signed transaction to storage nodes while it should be signed through the distributed workflow with multiple nodes.

Describe the solution you'd like

In the function Transaction.valid_stamps_signature? we should ensure the coordinator node is different from the cross validation nodes. This check should be done only if the validation node list has at least 2 nodes