Add helpers for txpool

[palango]

Resolves #72

Adds a new CeloContext as described in #72. Will need changes after #26 .

Open questions

• What should we do if a fee currency is whitelisted but we cannot get an exchange rate?
• Can we use the exchange rates available in the BlockContext?

[karlb]

What should we do if a fee currency is whitelisted but we cannot get an exchange rate?

Is there any real choice but not to include txs with that fee currency?

[palango]

What should we do if a fee currency is whitelisted but we cannot get an exchange rate?

Is there any real choice but not to include txs with that fee currency?

I'm asking because this is not what we currently do: https://github.com/celo-org/celo-blockchain/blob/d2e7c9f3a44a727a8117da9c928a3ccb8e582f27/contracts/currency/currency.go#L206

But this behaviour doesn't make sense to me either and I think we should avoid those transactions.

[hbandura]

Maybe the "IsWhitelisted" should be something like "IsAvailable", and if we cannot find the exchange rate OR if it's not whitelisted, we should drop all txs

[palango]

Maybe the "IsWhitelisted" should be something like "IsAvailable", and if we cannot find the exchange rate OR if it's not whitelisted, we should drop all txs

It behaves like that now. But I still think IsWhitelisted fit's better.

[palango]

Included in #43