Async name_to_address middleware

[kclowes]

What was wrong?

The name to address middleware relies on calls to an RPC server. In order to have a full async pathway, web3py needs an async version of the middleware that will use an async w3 instance.

How can it be fixed?

Add an async_name_to_address_middleware that can be used in the async pathway. This issue builds on #1986, and will depend on #1987.

[dbfreem]

So I started working on this the other day since I felt like it was one of the shortcomings of the Async Provider currently. There were a couple of module level methods that needed to be asynced in contracts to call the eth.call async method. Also, I was thinking of creating an AsyncContractFuntion. This wouldn't be a full implementation of ContractFunction in Async but just enough to get bye to make the ENS contract call. Also, was thinking of doing it in pieces. One PR for AsyncContractFunction, one for ENS methods needed for the middleware, and one PR for the middleware itself. This might make it more manageable and easier to review the PRs.

@kclowes or anyone else have any thoughts on this approach.

[kclowes]

@dbfreem that approach sounds like what I would do as well, thanks for taking it on! Smaller PRs are preferred since they tend to be so much easier to review, and breaking it up like you suggest makes sense to me.

/cc @pacrob since you've been working on an async Contract