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/client/src/builder.rs

@@ -157,7 +157,10 @@ where
         let context = runtime_context.service_context("beacon".into());
         let spec = chain_spec.ok_or("beacon_chain_start_method requires a chain spec")?;
         let event_handler = if self.http_api_config.enabled {
-            Some(ServerSentEventHandler::new(context.log().clone()))
+            Some(ServerSentEventHandler::new(
+                context.log().clone(),
+                self.http_api_config.sse_capacity_multiplier,
+            ))
         } else {
             None
         };