OPCode/QueryID 6 should check the latest Blockhash and timestamp

[michelbarnich]

To prevent the Network from de-syncing completely, when a Node fails to accept a new Block, the QID/OPCode 6 should also request the newest Blockhash. This way if a Node didnt accept the newest Block, it will automatically be removed from the network, and not hinder the Network in accepting and validating new Blocks.

[michelbarnich]

I created a first implementation for this in commit 27e4380. Stability needs to be tested in the next few test runs. It should be just as stable as previous OPCode 6.

[michelbarnich]

When a Node was rejected by the Network, and it goes offline, it may cause a desync in the Network. I am not 100% sure if this is the cause though.

[michelbarnich]

The reachability check will now be executed 30 seconds after each full minute (commit f83818d), the desync was probably caused by the check happening at a inconvenient moment in time (one Node already rejected another, while others havent realized yet)