RFR: 8373409: java/net/httpclient/http3/H3ErrorHandlingTest.java failed due to deadlock

[Daniel Jeliński]

This PR fixes a deadlock between the `localConnectionIdManager` and the `connections` map by closing the manager before calling `connections.compute`.

No new tests; the issue requires a complex setup to reproduce, and the new code is easy enough to reason about. Existing tests continue to pass.

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Commit messages:
 - Reorder QuicConnectionImpl check first
 - Add bug ID
 - Avoid calling methods in QUIC package from CHM.compute

Changes: https://git.openjdk.org/jdk/pull/28788/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28788&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373409
  Stats: 57 lines in 2 files changed: 21 ins; 26 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/28788.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28788/head:pull/28788

PR: https://git.openjdk.org/jdk/pull/28788