coinbase's scriptSig content
    "key" : "hex",                         (string) values must be in the coinbase (keys may be ignored)
    ...
  },
  "coinbasevalue" : n,                     (numeric) maximum allowable input to coinbase transaction, including the generation award and transaction fees (in satoshis)
  "longpollid" : "str",                    (string) an id to include with a request to longpoll on an update to this template
  "target" : "str",                        (string) The hash target
  "