merging two issues can cause events to not be reflected in total event count ("times_seen")

## Important Details

This affects all Sentry versions, like with #25672 the environment information is irrelevant.

## Description

See #25672 for an introduction to event counts and what they are. This is again about `times_seen`.

## Steps to Reproduce

1. Make sure Sentry's `buffers` backend debounces incrs aggressively (not necessarily the case in development)
2. Have two issues younger than 90d, which continue to get some events every second.
3. Merge them together.
4. The new issue will have `times_seen` that is smaller than the amount of events currently retained, which is nonsense. So issuestream will show a higher count than issue details page does.

### What you expected to happen

`times_seen` on an issue should always be greater than or equal to the amount of events retained in ClickHouse, modulo temporary data inconsistencies.

### What probably happens under the hood

1. group `b` continues to get events and schedules debounced updates for `times_seen` and `last_seen` via buffers a couple mins into the future
2. group `b` is merged into `a`
3. buffers processes, and cannot find the group ID, incrs are discarded