Protecting envoy from stacking events on the same fd

*Description*:
The way envoy FileEventImpl use libevent is tricky. If two FileEventImpl refer to the same fd, eventually the assigned events are stacked
and it's probably hard to recover the intention of each other.

https://github.com/envoyproxy/envoy/pull/8922 is the one I encountered but I believe there are more hidden.

We need to figure out
1. what is pattern we should follow all over envoy,
2. how to detect the violation
3. what are the tools to debug when we suspect there is violation