```
// unbuffered
out, err := httpClient.Subscribe(ctx, "event.type=NewTx and account.name=Jack", 0)
// buffered
out, err := httpClient.Subscribe(ctx, "event.type=NewTx AND account.name=Jack", 20)
```
Before: when the `out` channel is buffered and becomes full, we drop an event (+ log the error)
After: when the `out` channel is buffered and becomes full, we block
**Before it was not apparent to the app when an event was dropped (looking at the logs is manual task). After this PR, if the user does not read from `out` on 1 subscription, all other subscriptions will be stuck too.**
Closes#6161