Rpc multiquery

[juampiq6]

Multiquery RPC feature:

This PR adds functionality for making several rpc queries in one request. Handling most of the stuff internally in a separate implementation of a Web3Client for compatibility purpose. I can add documentation for it in the main README.md file for people who need it as it can be a useful feature for a lot of people to ask once and get several responses (reducing latency times specially for dapps that make a lot of request). I think this is a key feature. I added some integration test but a lot more can be added (i saw the other parts of this library werent thoughly tested so i want make sense to keep a good test suite only for this feature). Hope this helps!

Any improvement is more than welcome as any corrections or discussion about the implementation! @xclud i hope you have time to review it, I have marked in the PR comments the part where it does actually change a little the library API, but so far the rest and the main functionality is developed in other implementation of the Web3Client.

[juampiq6]

@xclud all fixes done, i dont know what could be missing

[xclud]

Thank You @juampiq6,

There are a couple of merge conflicts.