Change the gas refund rules

Change the gas refund rules to the following: Full refund for a bond with min. value of $1000 would mean min. 500$ LP + 500$ sync. Therefore the 100% refund should count for LP of 500$ and not for 1000$.

I suggest to change this point: -Minimum Value added as liquidity for a full refund is $500. -if Liquidity added value < $500 but > $200 a 25% refund is eligible.