feat: introduce return value for updatePrice to facilitate upkeep

[xenide]

While writing the keeper bot to update price ourselves as a last resort (after a period of unclaimed bounty), I realized one thing that needs improvement:

• there is no easy way to simulate if a price update will pay a bounty.
  • in the previous iteration, keeper would require rpc infra, which simulates an updatePrice call, and then see if the receiving wallet indeed has received a reward
  • this is quite involved and not just simulating a call and reading its return value

So the proposed solution here is to add a return value such that:

• MEV bots and our keeper can easily do a simulated call off chain to see if prices have indeed deviated beyond the threshold

Considerations

• I've thought of returning the result as a bool, but decided that a uint256 would be more useful for bots to ascertain the exact rewards applicable

Other improvements

• consolidate the ETH transfer in one call instead of transferring the bounty for each hop
• fixed a bug where in multiple-hop routes, the threshold for each hop wasn't read

[OliverNChalk]

Looks good, i do think mev bots should simulate to get the ETH transfer amount (check post balances) but this change looks good