Reject blocks if 15 mins outside network time

The limit before was 2 hours outside of the network time which is too
great with the faster retarget that Neutrinocoin has. 15 mins
significantly reduces the impact that a time warp attack can have on the
retarget.
Reject all non-V2 blocks

All blocks should be V2 which has nHeight. This explicitly rejects any
blocks that are not V2 or do not have nHeight.