Problem: missing support of sign batch

[leejw51crypto]

add function to send a lot of txs easily

1. sign txs as batch
2. send by sync or async way

[linfeng-crypto]

did some work to build the multisig StdTx, as same as in the chain-maind cli, but error in broadcast:
{'jsonrpc': '2.0', 'id': -1, 'error': {'code': -32603, 'message': 'Internal error', 'data': 'runtime error: index out of range [0] with length 0'}}
the StdTx is:

{
  "tx": {
    "msg": [
      {
        "type": "cosmos-sdk/MsgSend",
        "value": {
          "from_address": "cro1vcyncflzgqd3da3h68ndyjcn2kgt84y7jjtcee",
          "to_address": "cro1hk220qwxp0c8m3pzazardmmfv8y0mg7ukdnn37",
          "amount": [
            {
              "denom": "basecro",
              "amount": "100"
            }
          ]
        }
      }
    ],
    "fee": {
      "amount": [],
      "gas": "200000"
    },
    "signatures": [
      {
        "pub_key": {
          "type": "tendermint/PubKeyMultisigThreshold",
          "value": {
            "threshold": "2",
            "pubkeys": [
              {
                "type": "tendermint/PubKeySecp256k1",
                "value": "A6hVUjlwIwFFAQ38m7xg7tS3f4IlCZN+PVH1/lSa75NH"
              },
              {
                "type": "tendermint/PubKeySecp256k1",
                "value": "Ahc9WzVkqLNy4s11D96XGGASG9BeMlrPN7E7YvuOhwu2"
              }
            ]
          }
        },
        "signature": "CgUIAhIBwBJAttB4rkFW/MwMXgv9dUjO5P8qEoFWMhepicEFzDcfB+Ic5HjTr+NfTn7x2XquJr/sEvYknJQK25BXmsnEmHhGEhJAXYuiAJTf4yfnHg6SWA3/NzYULogroPGquZKSlXDW80w/jHJxzhU0NmPWlJFyKJeDsduovHPMG+c021Yw/hJxDg=="
      }
    ],
    "memo": "",
    "timeout_height": "0"
  },
  "mode": "|sync"
}