Make request to esplora in parallel

[RCasatta]

At the moment the Esplora blockchain source, used in our playground, is making non-blocking requests but they are not parallelized.
This allows specifying the desired concurrency level when using this blockchain source.
I went for a default of 4, which is pretty conservative but library users and our repl could obviously use a custom value.

Test with --esplora_concurrency X

RUST_LOG=info cargo run --release --example repl --features cli-utils,electrum,esplora -- --esplora https://blockstream.info/testnet/api --esplora_concurrency 10 --wallet change_deep --descriptor "wpkh(tpubD6NzVbkrYhZ4WrmVQXmpVcPoh1LUtuBaVLJ3PDPNEaFpw5iiBW9GwkMmtkpT3ucpN5G64Tfj9EAGVGkXrYFrpdBdvp8RMDaKhtoEp5cBDWY/0/*)" --change_descriptor "wpkh(tpubD6NzVbkrYhZ4WrmVQXmpVcPoh1LUtuBaVLJ3PDPNEaFpw5iiBW9GwkMmtkpT3ucpN5G64Tfj9EAGVGkXrYFrpdBdvp8RMDaKhtoEp5cBDWY/1/*)" sync  --max_addresses 200

concurrency	time for first sync
1	59s
4(default)	16s
10	8s

[RCasatta]

In theory, FuturesUnordered could squeeze performance a little bit, but I thought the increased code complexity wasn't worth the effort

[justinmoon]

Might be a good idea to just put "https://blockstream.info/testnet/api" as first argument so people can copy-paste from the documentation.

[justinmoon]

I'm getting this error when I try to run the example command you provided:

error: Found argument '--esplora_concurrency' which wasn't expected, or isn't valid in this context
        Did you mean --esplora?

It works if I update cli.rs like this:

    if cfg!(feature = "esplora") {
        app = app
            .arg(
                Arg::with_name("esplora")
                    .short("e")
                    .long("esplora")
                    .value_name("ESPLORA")
                    .help("Use the esplora server if given as parameter")
                    .takes_value(true),
            )
            // BELOW THIS IS NEW
            .arg(
                Arg::with_name("esplora_concurrency")
                    .long("esplora_concurrency")
                    .takes_value(true)
                    .help("Concurrent requests to esplora backend"),
            );
            // ABOVE THIS IS NEW
    }

Also seems like CI is stuck ...

[RCasatta]

Oh thanks, it looks I forgot the last push :)

[justinmoon]

Looks good to me. With cli update it compiles and I see a similar speedup.

[afilini]

ACK c9079a7