Block->miner: resolve into an EthqlAccount

[raulk]

Support these kinds of queries:

block(number: 5000000) {
  miner {
    address
    balance
  }
  parent {
    miner {
      address
    }
  }
}

transaction(hash: "0x....") {
  block {
    miner {
      address
      transactionCount
    }
  }
}