Fixed bug in voting

[panleone]

(For some Reason git closed the old PR while I was force pushing a new version.)

With the current system users can update their vote to a given proposal with the condition that the new vote is sent at least 1 hour after the old vote, this can lead to a possible vulnerability:

Let's consider the following scenario: a malicious node generates and propagates two votes (x,y), with x vote and y time stamp, to the same proposal for example A=(YES, 15:00) and B=(NO, 15:01).
What happens in this case to consensus? There are two possibilities:
If a node receive first the vote A and then the vote B it will accept A and reject B since the time difference is only one minute.
If a node receive first the vote B and then the vote A it will accept B and reject A since the time difference would be -1 minute.

Hence in this scenario nodes will not agree on final votes and this would lead to a fork at the superblock.

With this pull request I want to fix this vulnerability with the following:

• Nodes will always accept and relay a new vote only if it is submitted at least in the next 1 hour slot
• If the new vote is submitted in the same 1 hour slot of the old vote by convention we keep and relay the YES (relay only if the old vote was not YES to avoid spam)

[Liquid369]

tACK 1512dfa

I like this better than just before allowing if the vote is older than the last