Support pre-flight CORS check (#1772)

## Issue Addressed

- Resolves #1766 

## Proposed Changes

- Use the `warp::filters::cors` filter instead of our work-around.

## Additional Info

It's not trivial to enable/disable `cors` using `warp`, since using `routes.with(cors)` changes the type of `routes`.  This makes it difficult to apply/not apply cors at runtime. My solution has been to *always* use the `warp::filters::cors` wrapper but when cors should be disabled, just pass the HTTP server listen address as the only permissible origin.

validator_client/src/cli.rs

@@ -131,8 +131,10 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
             Arg::with_name("http-allow-origin")
                 .long("http-allow-origin")
                 .value_name("ORIGIN")
-                .help("Set the value of the Access-Control-Allow-Origin response HTTP header.  Use * to allow any origin (not recommended in production)")
-                .default_value("")
+                .help("Set the value of the Access-Control-Allow-Origin response HTTP header. \
+                    Use * to allow any origin (not recommended in production). \
+                    If no value is supplied, the CORS allowed origin is set to the listen \
+                    address of this server (e.g., http://localhost:5062).")
                 .takes_value(true),
         )
 }