tls: fix connection close on listener close

[DheerendraRathor]

In serveConnection, closing the muxed connection instead of underlying net.Conn to properly release resources. Muxed connection can be *tls.Conn and should be closed properly.

Other minor change is close the stoppedChan in serve at the end as closing it will unblock read in Close and will effectively close the listener. This should be the last logical instruction during close.

• [x] Description and context for reviewers: one partner, one stranger
• [x] Docs (package doc)
• [] Entry in CHANGELOG.md Minor change, skipping entry in CHANGELOG

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[codecov[bot]]

Codecov Report

Patch coverage: 50.00% and project coverage change: -0.01 :warning:

Comparison is base (e3ff3d8) 85.35% compared to head (5c18790) 85.34%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #2206 +/- ## ========================================== - Coverage 85.35% 85.34% -0.01% ========================================== Files 270 270 Lines 15475 15475 ========================================== - Hits 13208 13207 -1 - Misses 1845 1846 +1 Partials 422 422 ``` | [Impacted Files](https://codecov.io/gh/yarpc/yarpc-go/pull/2206?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yarpc) | Coverage Δ | | |---|---|---| | [transport/internal/tls/muxlistener/listener.go](https://codecov.io/gh/yarpc/yarpc-go/pull/2206?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yarpc#diff-dHJhbnNwb3J0L2ludGVybmFsL3Rscy9tdXhsaXN0ZW5lci9saXN0ZW5lci5nbw==) | `93.87% <50.00%> (-2.05%)` | :arrow_down: | ... and [2 files with indirect coverage changes](https://codecov.io/gh/yarpc/yarpc-go/pull/2206/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yarpc) Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yarpc). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yarpc)

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