fix(protocol): limit reward per gas in weight calculation

[dantaik]

About

Previously a prover can set the expected rewardPerGas really small to increase his own weight while still able to receive 80% * feePerGas as the effective rewardPerGas.
With this change, a prover's effective rewardPerGas is in the range of [80%-120%] * feePerGas. This effective rewardPerGas will also be returned from ProverPool to LibProposing.

Thanks to https://github.com/hugo-blue for reporting this issue.

Also, feePerGas and rewardPerGas shall be uint32, not uint16.

Gas

Previously:

Deployment Cost	Deployment Size
1680364	8425
Function Name	min	avg	median	max	# calls
assignProver	15329	18128	17264	81329	463
stake	29849	73572	76569	169703	151

Now:

Deployment Cost	Deployment Size
1683164	8439
Function Name	min	avg	median	max	# calls
assignProver	14406	21530	19052	80406	463
stake	29672	73229	76313	169383	151