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/beacon_chain/src/events.rs

@@ -21,8 +21,11 @@ pub struct ServerSentEventHandler<T: EthSpec> {
 }
 
 impl<T: EthSpec> ServerSentEventHandler<T> {
-    pub fn new(log: Logger) -> Self {
-        Self::new_with_capacity(log, DEFAULT_CHANNEL_CAPACITY)
+    pub fn new(log: Logger, capacity_multiplier: usize) -> Self {
+        Self::new_with_capacity(
+            log,
+            capacity_multiplier.saturating_mul(DEFAULT_CHANNEL_CAPACITY),
+        )
     }
 
     pub fn new_with_capacity(log: Logger, capacity: usize) -> Self {