Support consistent reads (`call`-s) at a specified block number

It's not possible to execute read-only methods (the ones that execute a call underneath) consistently if the node we are connected to is constantly importing new blocks. It becomes a problem during long-running operations as we may end up with inconsistent state.

Solution would be to introduce reads at a specific block number.

We probably have to wait until ethereum/web3.py#446 gets implemented, otherwise it would be a bit too much hacking...?