Update the documentation with number of maximum validators

[akolotov]

There are few places in the code where the list of validators is iterated:

https://github.com/poanetwork/tokenbridge-contracts/blob/20d262702defd3aed39870f3379f1cf0c314305e/contracts/upgradeable_contracts/BaseBridgeValidators.sol#L58-L76

and

https://github.com/poanetwork/tokenbridge-contracts/blob/fdaab2777efe2f48c20a690c9f59196fa89e3631/contracts/upgradeable_contracts/ValidatorsFeeManager.sol#L23-L54

in these methods excessive number of validators could lead to "out of gas" issue.

Consider to do the following:

• perform a research and update the documentation with maximum number of validators which will not lead to the transaction interruption;
• update setValidatorCount to limit the number of bridge validator that can be added;
• update tests that will confirm that the number of invocations of _addValidator that corresponds to the recommended maximum of validators does not exceed some default gas limit (e.g. gas limit for the Ethereum Mainnet)
• update tests for the initialize methods in both RewardableValidators and BridgeValidators that will confirm that passing the validators list with size corresponding to the recommended maximum of validators does not exceed the default gas limit.

[k1rill-fedoseev]

I performed some kind of a research. I have tested contracts in Istanbul and Petersburg environments, considering 8000000 as a gas limit for a transaction.

I have came up with the following upper bounds for following contracts:

• BridgeValidators
  • 287 validators in Istanbul
  • 300 validators in Petersburg
• RewardableValidators
  • 156 validators in Istanbul
  • 161 validators in Petersburg
• Erc to Native home bridge
  • 154 validators in Istanbul (bounded by executeAffirmation call)
  • 140 validators in Petersburg (bounded by executeAffirmation call)
• Erc to Erc home bridge and Native to Erc home/foreign bridge
  • Calls involving fees distribution require less gas then initialise call, so upper bound on validators is 156

[akolotov]

Great! So, let's have a limit in 50 validators.