BDIP18 (Formal vote): Implement rewards vesting

This is similar to the current proposal under consideration by https://mith.cash/ This is also similar to implementations done by Sushiswap and Luaswap.

Basically, the idea behind this: Regardless of where BSD price is (whether below $1, or above $1), all rewards should have a vesting component.

By having a vesting component, this reduces selling pressure of BSD and BSDS on the market.

A straightforward implementation: 25% of rewards available immediately; and 75% of rewards available after 6 months, and vests evenly for 6 months after the lockup expires.

Advantages:

1. Reduces selling pressure of BSD and BSDS on the market.
2. The vesting period means people will stay engaged longer (so as to claim rewards).

Disadvantages:

1. Reduces APY which may mean less interested farmers (but then again, you will have avoided the speculative farmers).