Do not ask for the first hash if GlobalTasks

[vncoelho]

@erikzhang,
There was no verification for the first hash.

I included this else:

                if (!globalTasks.Contains(hash))
                    session.RemoteNode.Tell(Message.Create("getblocks", GetBlocksPayload.Create(hash)));
                else
                    Console.WriteLine("Wrong behavior. Asking for a hash contained in global task");

And the message appeared.

[jsolman]

Hmm if the hash is in globalTasks it could be getting data for the block, but this is asking for inventory and since this doesn’t give hash stop it will potentially get newer block inventory, so this could still be useful to leave the way it is actually.

[jsolman]

It is desirable behavior to have it learn about newest blocks from whichever is the latest on, so I think you could close this PR.

[vncoelho]

It looks like you are right again, @jsolman.

It kind of always reply with 500 blocks (which is not desirable from my perspective).
In this sense, I still think that the plausible possibility I thought was to ask for blocks in windows ( as I proposed in a previous PR, with a loop #486).