Parallelize batches

[LesnyRumcajs]

Summary

According to JSON-RPC 2.0 spec

The Server MAY process a batch rpc call as a set of concurrent tasks, processing them in any order and with any width of parallelism.

However, we use jsonrpsee's defaults which don't apply any parallelism. So a batch of 100 eth calls (pretty common scenario) has to get through each call in the batch one by one. Most inefficient!

Completion Criteria

• Parallelize batch requests. Worth adding a sane concurrency knob. This should be possible without forking jsonrpsee via overrides in the middleware.

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