parachain: Block production time for DataHighway parachain must match the relay chain

[ltfschoen]

As discussed in this comment of a PR for the DataHighway parachain, we learnt from this post by the Picasso team that they encountered an issue where their parachain stopped producing blocks https://kusama.polkassembly.io/proposal/53.

It was caused because they used their standalone chain blocktime instead of the parachain standard blocktime.
So this would affect us too, we're re-calculated how this affects things in the (internal) "DHX Mining & Halving" spreadsheet, as the "Block Production / Mining Time" input would change, which affects the % of DHX total supply issued each year, and it affects other outputs since if we instead used a block time of 12s instead of 4.32s, then the "Block Reward Issuance Per Day (DHX)" would change from 5000 DHX to 1800 DHX.

So if we change the block time from 4.32s to 12s, then to overcome these issues we need to increase the block reward from 0.25 to ~0.694445 to maintain 5000 DHX block reward distribution per day