Add batching of transactions for opening a channel & depositing

[loredanacirstea]

We can have a wrapper for opening a channel and depositing. This lowers the waiting time for the user, as it reduces the number of on-chain transactions from 2 to 1.

[hackaugusto]

This not only improves the wait time, but it also simplifies the handling of smart contract proxies. Specifically the ERC20 token networks a client has to:

• Use a global lock to synchronize usage to a token and to its corresponding token network. This is necessary because calls to approve are not idempotent, and concurrent calls will race, making one of them fail.
• It's necessary to validate multiple conditions before doing a deposit, on both the token and the token network state, and the interactions lead to cumbersome code.

reference

[pirapira]

Note: consider implementing the proxy in a separate contract. We are close to hitting the deployed codesize limit.

[palango]

Closed in #1436