The POH smart contract calls might reach the limit

[JeremyPansier]

Context
The call to POH smart contract is currently done via infura using the same key for all nodes. The infura free plan is limited to 100,000 requests a day that implies that the current implementation will reach the limit.

• Add a program argument for the infura API key
• Use it to call the POH smart contract
• Replace infura key

[JeremyPansier]

This issue might be the opportunity to use websockets to call the smart contract:

const pohSmartContractAddressHex = "0xC5E9dDebb09Cd64DfaCab4011A0D5cEDaf7c9BDb"

type Registry struct {
	clientUrl string
}

func NewRegistry(infuraKey string) *Registry {
	clientUrl := fmt.Sprintf("https://%s.infura.io/v3/%s", "mainnet", infuraKey)
	return &Registry{clientUrl}
}

func (registry *Registry) IsRegistered(address string) (isRegistered bool, err error) {
	client, err := ethclient.Dial(registry.clientUrl)
	if err != nil {
		return
	}
	pohSmartContractAddress := common.HexToAddress(pohSmartContractAddressHex)
	proofOfHumanity, err := NewPoh(pohSmartContractAddress, client)
	if err != nil {
		return
	}
	isRegistered, err = proofOfHumanity.PohCaller.IsRegistered(nil, common.HexToAddress(address))
	if err != nil {
		return
	}
	return
}

[JeremyPansier]

protect infura key by restricting origin if possible... see https://community.infura.io/t/status-code-403-forbidden-not-allowed-invalid-origin/7000