API - provide hashes for all on-chain transactions

[manuelwedler]

Motivation

At the moment all API calls that perform on-chain transaction block and only return after the transaction was mined. For providing better feedback to the user (e.g. etherscan link, raiden-network/webui#635), it would be nice to have a way to get the transaction hashes returned after such an API call was done.

Specification

After making an API call, which triggers an on-chain transaction, the transaction hash should be returned. It should then still be possible to have a way to get notified when the transaction was finalized.

The details depend on how we implement the asynchronous API in general: #5455

Backwards Compatibility

This should be part of a API v2 (#6543)