fix(server): actually perform shutdown on connection loss

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: +0.11 :tada:

Comparison is base (bb19bdf) 83.10% compared to head (8f13f34) 83.21%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #90 +/- ## ========================================== + Coverage 83.10% 83.21% +0.11% ========================================== Files 11 11 Lines 728 727 -1 ========================================== Hits 605 605 + Misses 123 122 -1 ``` | [Impacted Files](https://app.codecov.io/gh/JKRhb/dtls2/pull/90?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jan+Romann) | Coverage Δ | | |---|---|---| | [lib/src/dtls\_server.dart](https://app.codecov.io/gh/JKRhb/dtls2/pull/90?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jan+Romann#diff-bGliL3NyYy9kdGxzX3NlcnZlci5kYXJ0) | `87.66% <ø> (+0.38%)` | :arrow_up: |

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