jsonrpc: small refactor, extract method for groupedJSON-RPC calls.

[msf]

this is a small improvment to the current ugly code we use for:

• grouping error handling of rpc calls for the same block
• abiding to a maximum concurrency limit for the whole RPC node.

This is ontop of #70,

[adammilnesmith]

Wondering what you think of having a function like this:

func (c *rpcClient) execOnPoolInGroup(
	group *errgroup.Group,
	function func() error,
) {
	group.Go(func() error {
		errCh := make(chan error, 1)
		c.wrkPool.Submit(func() {
			defer close(errCh)
			err := function()
			if err != nil {
				errCh <- err
			} else {
				errCh <- nil
			}
		})
		return <-errCh
	})
}

That can be called like this:

c.execOnPoolInGroup(group, func() error {
	err := c.getResponseBody(ctx, method, args, output)
	if err != nil {
		c.log.Error("Failed to get response for jsonRPC",
			"blockNumber", debugBlockNumber,
			"method", method,
			"error", err,
		)
	}
	return err
})

[msf]

I like it, but don't we end up writting more core? we always want to call a jsonRPC method and including the err handling log.
Or said in a different way, I think that is a better abstraction to hide the mess of group.Go() + wrkPool.Submit() but I also want the syntatic sugar to run the prebaked closure you wrote above.

maybe: "porque no los dos?" -- we can have both, let me do that.