contract, swap: refactor backend to commit on sendTransaction

[ralph-pichler]

This PR introduces the chain.WaitMined function which supersedes the global contract.WaitFunc (which was used for testing and modified by the setupContractTest helper function) and the bind.WaitMined function.

During tests , the simulated backend is wrapped in a chain.TestBackend which commits after a successful SendTransaction. All backend specific stuff was moved from the contracts/swap package to the new swap/chain package which will contain the transaction queue in a follow-up PR.

bind.WaitMined is no longer used as it required a types.Transaction which might not be available. Instead a slightly modified version thereof is included in the chain package. The setupContractTest is only used for the cash cheque function which will be removed in the next PR, afterwards setupContractTest will be dropped.

[Eknir]

Functionally equivalent. Refactor by putting the backend logic to a separate file.

From the PR description, it is not immediately clear how and why this modification was needed, but I assume a follow-up PR comes where the purpose of this refactor will become clear.

I added one comment about the Close function which is not implemented. Please address this one before merging.

[ralph-pichler]

@Eknir regarding the why this was needed:

• the current way of testing by modifying global variables (WaitFunc and cashCheque) is very suboptimal. It needs the setupContractTest helper which adds clutter to tests and can have strange behaviour if used recursively with other helper functions which need this. With this change half of those problems go away (and with the follow-up PR setupContractTest will be retired completely).

• bind.WaitMined was replaced because it only takes a types.Transaction which is quite difficult to obtain if you didn't send the transaction from the running code (e.g. after a restart). The new function makes tracking transactions by hash much easier.

• the reason for the package move is that swap/chain or whatever it is going to be called in the end, will be the single component responsible for tracking transactions, therefore it makes sense for things like backend and wait functions to be defined there.

[acud]

build is failing https://travis-ci.org/ethersphere/swarm/jobs/648363173?utm_medium=notification&utm_source=github_status

[acud]

few more minor things but otherwise LGTM