Remove synchronous web3.js calls

[frangio]

We found one usage of web3.js without a callback, which results in a blocking call.

zos-lib/src/utils/Proxy.js

Line 21 in c7e3254

return web3.eth.getStorageAt(this.address, position)

We want to use asynchronous calls everywhere, otherwise for example we won't be able to implement #172. We should find all instances where web3.js is being used synchronously and add a callback, or consider using a promisified version of web3.js (buidler's?).

[s1na]

I'd like to give this a try, if it's okay with you guys.

[facuspagnuolo]

For sure @s1na, thanks!

[spalladino]

Hey @s1na! Please keep us posted on how it's going with this issue, and if you need any help. Thanks!

[s1na]

@spalladino Hi! Sry it's been some time, I was struggling with #206 and didn't make much progress on this. Please let me know if this is high priority so that either I can spend more time on it, or let someone else work on it instead.