Add timelock for Sushi rewards

As of right now, the incentives for providing liquidity to the pools do not guarantee that the liquidity will permanently migrate to Sushi Swap. To ensure the longlasting success of Sushi Swap, I propose that rewards be locked until one week of trading has been available. This way, the liquidity for traders is guaranteed on day 1, no major dump when liquidity mining cools off and sushi rewards go to the long term believers. Sushi can then demonstrate its ability to be the go-to AMM and the incentives for holding Sushi become much stronger. Vote yes for a timelock on rewards.