Gracefully handle contracts before starting block

[0xJem]

We don't currently have a clean way of handling contracts prior to their starting block:

• ERC20 contracts have a defined starting block constant, and the ERC20 helper function will return null if prior to that (which prevents further operations). This requires manual configuration, which isn't ideal.
• Liquidity pairs don't have any such protection

Additionally, there is plenty of code (e.g. getUSDRateUniswapV3) that expects a contract to bind and work after that. Error conditions (such as being before the starting block) should be handled gracefully, instead of throwing an error.