Use nodejs worker for block fetching

[ianhe8x]

to solve the process hang issue when processing a large ws payload

[fishmonger45]

I've been working on this a little bit today (https://github.com/wqsz7xn/subql/tree/node-worker). It's fairly simple and just means that you have to offload fetchBlockBatches to a nodejs worker. The only problem that i'm experiencing is that you have to pass the api, block numbers and overallspecVersion to the worker and doing that is difficult without some amount of serialisation (worker.postMessage only accepts some primitives/is limited). This would probably have a hit on performance.

Workers also require a worker file which doesn't support typescript obviously which is proving a bit tricky