Use @metamask/eth-query instead of Ethers

[mcmire]

It seems that the recent change to switch from ethjs-query to Ethers (#39) has caused a regression: when the network is switched, this.web3 still points to the previous network. This happens because Web3Provider saves a copy of the sendAsync method on the given provider, so even if nonce-tracker is given a provider proxy, this.web3 still has a reference to the sendAsync method from the previous provider object. (This was reported in Slack — see the shared libraries channel.)

This is a common problem with Ethers. Although Ethers has some nice features, we probably don't need to pull it in here, and we can use @metamask/eth-query (which provides TypeScript support, unlike @metamask/ethjs-query) instead.