Fix unresponsiveness during benchmark execution

[joao-paulo-parity]

In paritytech/polkadot#3862 we saw the bot not being responsive. That happened because a benchmark was being executed in the meantime and, since we're executing the commands synchronously, the main thread becomes blocked until the command is finished, therefore the bot is not able to respond to new requests.

bench-bot/bench.js

Line 37 in 992a8af

const result = shell.exec(cmd, { silent: false })

We do not want to block the main thread but instead stay responsive and continue accepting new requests while a benchmark is currently being executed.

The overhead from using asynchronous Node.js APIs might be an unsolvable problem and thus we'd have to rewrite the bot in a more performant language.