Add TLS capability to the beacon node HTTP API (#2668)

Currently, the beacon node has no ability to serve the HTTP API over TLS.
Adding this functionality would be helpful for certain use cases, such as when you need a validator client to connect to a backup beacon node which is outside your local network, and the use of an SSH tunnel or reverse proxy would be inappropriate.

## Proposed Changes

- Add three new CLI flags to the beacon node
  - `--http-enable-tls`: enables TLS
  - `--http-tls-cert`: to specify the path to the certificate file
  - `--http-tls-key`: to specify the path to the key file
- Update the HTTP API to optionally use `warp`'s [`TlsServer`](https://docs.rs/warp/0.3.1/warp/struct.TlsServer.html) depending on the presence of the `--http-enable-tls` flag
- Update tests and docs
- Use a custom branch for `warp` to ensure proper error handling

## Additional Info

Serving the API over TLS should currently be considered experimental. The reason for this is that it uses code from an [unmerged PR](https://github.com/seanmonstar/warp/pull/717). This commit provides the `try_bind_with_graceful_shutdown` method to `warp`, which is helpful for controlling error flow when the TLS configuration is invalid (cert/key files don't exist, incorrect permissions, etc). 
I've implemented the same code in my [branch here](https://github.com/macladson/warp/tree/tls).

Once the code has been reviewed and merged upstream into `warp`, we can remove the dependency on my branch and the feature can be considered more stable.

Currently, the private key file must not be password-protected in order to be read into Lighthouse.

beacon_node/src/cli.rs

@@ -217,6 +217,29 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .help("Disable serving of legacy data on the /config/spec endpoint. May be \
                        disabled by default in a future release.")
         )
+        .arg(
+            Arg::with_name("http-enable-tls")
+                .long("http-enable-tls")
+                .help("Serves the RESTful HTTP API server over TLS. This feature is currently \
+                    experimental.")
+                .takes_value(false)
+                .requires("http-tls-cert")
+                .requires("http-tls-key")
+        )
+        .arg(
+            Arg::with_name("http-tls-cert")
+                .long("http-tls-cert")
+                .help("The path of the certificate to be used when serving the HTTP API server \
+                    over TLS.")
+                .takes_value(true)
+        )
+        .arg(
+            Arg::with_name("http-tls-key")
+                .long("http-tls-key")
+                .help("The path of the private key to be used when serving the HTTP API server \
+                    over TLS. Must not be password-protected.")
+                .takes_value(true)
+        )
         /* Prometheus metrics HTTP server related arguments */
         .arg(
             Arg::with_name("metrics")