Modify Ethereum plugin config to use Connections store

[krisbitney]

This PR modifies the Ethereum plugin's config to use a Connections store. The Connections class stores Connection instances. App developers can keep a reference to the Connections store and use it to update networks and providers without reconstructing the plugin or client.

Original spec: https://hackmd.io/@polywrap-dao/BkQdG7NAq

Implemented UX:

const connections = new Connections({
  networks: {
    mainnet: new Connection({ provider }),
    rinkeby: new Connection(...)
  },
  defaultNetowrk: "mainnet"
})

ethereumPlugin({
  connections
});

// assign a Connection to "ropsten" network, or replace existing connection if ropsten exists in store
connections.set("ropsten", new Connection({ provider: newProvider })); // pass a Connection
connections.set("ropsten", newProvider); // or pass a provider directly

// get a Connection and call setProvider to avoid constructing a new Connection instance
connections.get("rinkeby").setProvider(newProvider);

Feedback is welcome!

[dOrgJelli]

can this be removed?

[krisbitney]

Yes, of course. That was just a note for myself that I forgot to delete. It was related to something I was thinking of doing originally, before I realized it wouldn't work out.

[dOrgJelli]

I'm a bit concerned with how we don't have much testing that exhausts these new functions. Could we add a test file for connections + connection?

[krisbitney]

Yes, of course. I'll update with tests asap.

[krisbitney]

I've added tests for Connections and Connection

[krisbitney]

Added support for passing providers directly into connections.set, to match the original spec:

connections.set("ropsten", newProvider);

[dOrgJelli]

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