Implementing state to /prices endpoint in line with spec addition of D14267
/prices/ now returns a `validThru` keyThis approach simplifies implementing registration in wallets. Wallets already need to make an API call to see if an alias is available or not. This key provides a blockheight 1000 blocks in front ofIf it is not available, return the current block.
If alias registration prices changeprice (and the next block, this would be updated so that Cashtab users would not accidentally register an alias at a legacy priceand the blockheight this price is good thru).