No liquidation optimizations

[uchami]

• Remove extra transfer to MoC.sol
• Remove paidMoc and paidRBTC from storage, use an event instead
• Refactor: perform less calls between MoC.sol and MoCVendors.sol
• Use Soldity 0.5.17 with optimizer enabled
• Instead of doTransfer for comissions, only use transfer to avoid calling moc state
• Refactor: ask for Rbtc and Moc price only once in MoCExchange
• Remove MocConverter.sol and move it's logic to mocState, this greatly reduces the number of calls between contracts
• Fix staking amount condition to removeStaking