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/http_api/src/test_utils.rs

@@ -225,6 +225,7 @@ pub async fn create_api_server_on_port<T: BeaconChainTypes>(
             allow_sync_stalled: false,
             data_dir: std::path::PathBuf::from(DEFAULT_ROOT_DIR),
             spec_fork_name: None,
+            sse_capacity_multiplier: 1,
             enable_beacon_processor: true,
         },
         chain: Some(chain),