abci socket client: reqQueue fills up quickly

If you send many transactions to the mempool, tendermint's `reqQueue` (size: 256) fills up fast. https://github.com/tendermint/tendermint/blob/2fd5fa7b5dea82beedea2fd9288e2417038300a9/abci/client/socket_client.go#L21 When `reqQueue` fills up, `socketClient.CheckTxAsync` blocks, which blocks `mempool.CheckTx`, which blocks the main mempool reactor's `Receive`.