Proposal: don't support tx batch sending

Current behavior:

The client wants to send multiple transactions concurrently and expects them to be included in the block successfully eventually.
The problem is checkTx reject immediately if the nonce check failed, so if transactions from the same sender with increasing nonce arrived at the node in the wrong order, they'll be rejected.
To support this we can either:

• wait for abci++ to support sth similar to go-ethereum's non-executable transaction queue in tendermint
• support multiple MsgEthereumTx in the same tx.

In option 2 we need to be able to query eth transaction information by eth tx hash, currently we do that with /tx_search?query="ethereum_tx.ethereumTxHash=$ETH_TX_HASH", in batch tx case, we need to further iterate the result list to find the msg result, should be doable though.

Desired behavior: support multiple MsgEthereumTx in the same tx.

Use case: client send batch transactions

Requests may be closed if we're not actively planning to work on them.