… scheduled at ~approximately the same time (#3092)

#3091

The problem is we have an edge case where we should drain the timer
channel, but we "let it slide" in certain race conditions when two
timeouts are scheduled near each other. This means we can have unsafe
timeout behavior as demonstrated in the github issue, and likely more
spots in consensus.

Notice that aside from NewTimer and OnStop, all timer accesses are from
the same thread. In NewTimer we can block until the timer is drained
(very quickly up to goroutine scheduling). In OnStop we don't need to
guarantee draining before the method ends, we can just launch something
into the channel that will kill it.

In the main timer goroutine, we can safely maintain this "timerActive"
variable, and force drain when its active. This removes the edge case.

The test I created does fail on main.

---

#### PR checklist

- [X] Tests written/updated
- [x] Changelog entry added in `.changelog` (we use
[unclog](https://github.com/informalsystems/unclog) to manage our
changelog)
- [x] Updated relevant documentation (`docs/` or `spec/`) and code
comments
- [X] Title follows the [Conventional
Commits](https://www.conventionalcommits.org/en/v1.0.0/) spec

(cherry picked from commit 153281a)