Parallel execution of token approvals

Some contracts (e.g. Tub) need multiple approvals in order to be able to work, whereas some other ones (e.g. SimpleMarket) often need to be approved to access multiple tokens. It gets even more complex if a 'TxManager` is involved.

Unfortunately token approvals are sequential right now i.e. the keepers wait for the each previous approval to get mined before they send a next one. It means that the whole process can easily take 5-10 minutes.

Would be nice to be able to send multiple approvals in parallel, the same way it can be done with transact_async() for regular transactions.

It's not a high-priority task as approvals only happen the first time we start a keeper, so it's not such a big deal to wait these 5-10 minutes.