did:web resolver HTTP client creation is very slow

[vdods]

In optimizing my verification codepaths, I've noticed that a major bottleneck was the creation of a new reqwest::Client for each did:web DID doc resolution. It's mostly because loading up all the TLS certs is a pretty involved operation. The reqwest crate documentation ( https://docs.rs/reqwest/latest/reqwest/struct.Client.html ) says

The Client holds a connection pool internally, so it is advised that you create one and reuse it. You do not have to wrap the Client in an Rc or Arc to reuse it, because it already uses an Arc internally.

I fixed this issue in my own fork by creating a reqwest::Client singleton via lazy_static (example below), and then cloning that to make HTTP requests. In my particular verification-heavy codepath, this led to reducing the run time by over 60%!

This lazy_static approach is the easiest and laziest solution. However, if there are other crates that also do similar and have their own lazy_static reqwest::Client singletons, having a redundant one be less than ideal. I don't know a good approach for how to handle that. Maybe there could be an initialization function that is optionally called to set the reqwest::Client singleton to be a clone of the one supplied to that initialization function.

Anyway, because of how severe a bottleneck this is, I'd suggest we address this somehow in the did-web crate.

I put the following in did-web/src/lib.rs:

lazy_static::lazy_static! {
    /// Building a reqwest::Client is *incredibly* slow, so we use a global instance and then clone
    /// it per use, as the documentation indicates.
    pub static ref REQWEST_CLIENT: reqwest::Client = {
        let mut headers = reqwest::header::HeaderMap::new();

        headers.insert(
            "User-Agent",
            reqwest::header::HeaderValue::from_static(USER_AGENT),
        );

        let client = match reqwest::Client::builder().default_headers(headers).build() {
            Ok(c) => c,
            Err(err) => { panic!("Error building HTTP client: {}", err.to_string()) }
        };

        client
    };
}