Use more cores with rayon while syncing with the CDN

[ljedrz]

Since we perform the CDN sync before starting the node, we can use a rayon threadpool with all the cores dedicated to syncing without sacrificing any other functionality.

I tested it back in testnet3 and saw higher CPU utilization.

[howardwu]

How much faster is the rate of syncing? Can you share numbers for pre and post syncing for this PR?

[ljedrz]

Back when I was testing on testnet3 I noted that it doubled my CPU utilization; the exact increase in speed would depend on the number and power of cores, but this results in both faster deserialization and ledger insertion.

[howardwu]

Do you have empirical data that you can share to confirm this finding?

[ljedrz]

Unfortunately no, not unless @zosorock has performed some measurements back when this was requested.

In any case, it's a pretty trivial change, so it can wait until the next time we're using a CDN if we'd like to collect some solid figures before merging.