RFR: 8373409: java/net/httpclient/http3/H3ErrorHandlingTest.java failed due to deadlock [v2]

[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.

Daniel Jeliński has updated the pull request incrementally with one additional commit since the last revision:

  Reorder startTimer after map updates

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Changes:
  - all: https://git.openjdk.org/jdk/pull/28788/files
  - new: https://git.openjdk.org/jdk/pull/28788/files/d379634d..84f5e430

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28788&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28788&range=00-01

  Stats: 4 lines in 1 file changed: 2 ins; 2 del; 0 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