Replace low level `forkIO` with a safer alternative

[janmasrovira]

using forkIO is not recommended because:

async's high-level API spawns lexically scoped threads, ensuring the following key poperties that make it safer to use than using plain forkIO:

• No exception is swallowed (waiting for results propagates exceptions).
• No thread is leaked (left running unintentionally).

Recently we've observed some infinite loops when one of the threads was crashing and I suspect this could fix that issue. I don't have a concrete example to share.