Add another type of spork to invalid block by its hash or by its height

[hanzac]

This is to work in case of network having trouble of chain forks.
This can work with SPORK_6_RECONSIDER_BLOCKS: Forces reindex of a specified number of blocks to recover from unintentional network forks.

[TheSin-]

turns out that there is no code for RECONSIDER_BLOCKS SPORK at all, Dash must have removed it

and since as I mentioned you can only use int64s for SPORKS you can't send a hash, invalid can't be controlled that way.

I'm going to keep this open to think about it but it's likely not going to see it's way in anytime soon since the first spork needs to be written and the second one isn't a valid type.