Don't kill SSE stream if channel fills up (#4500)

## Issue Addressed

Closes #4245

## Proposed Changes

- If an SSE channel fills up, send a comment instead of terminating the stream.
- Add a CLI flag for scaling up the SSE buffer: `--http-sse-capacity-multiplier N`.

## Additional Info

~~Blocked on #4462. I haven't rebased on that PR yet for initial testing, because it still needs some more work to handle long-running HTTP threads.~~

- [x] Add CLI flag tests.

beacon_node/src/cli.rs

@@ -382,6 +382,15 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                     stalled. This is useful for very small testnets. TESTING ONLY. DO NOT USE ON \
                     MAINNET.")
         )
+        .arg(
+            Arg::with_name("http-sse-capacity-multiplier")
+                .long("http-sse-capacity-multiplier")
+                .takes_value(true)
+                .default_value("1")
+                .value_name("N")
+                .help("Multiplier to apply to the length of HTTP server-sent-event (SSE) channels. \
+                       Increasing this value can prevent messages from being dropped.")
+        )
         .arg(
             Arg::with_name("http-enable-beacon-processor")
                 .long("http-enable-beacon-processor")

beacon_node/src/config.rs

@@ -149,6 +149,9 @@ pub fn get_config<E: EthSpec>(
         client_config.http_api.allow_sync_stalled = true;
     }
 
+    client_config.http_api.sse_capacity_multiplier =
+        parse_required(cli_args, "http-sse-capacity-multiplier")?;
+
     client_config.http_api.enable_beacon_processor =
         parse_required(cli_args, "http-enable-beacon-processor")?;