Improve spammer to handle high TPS

[linoscope]

Right now, when you try to submit a lot of txs with high --batch-size value, we get:

Failed to establish a new connection: [Errno 24] Too many open files'))

Alternatively, if I set lower value for --sleep (like 0.1), seems like the each batch is taking longer than 0.1 hence not increasing TPS much.