Fix concurrency issue on connection tracker

[calvin681]

Context

There can be a race condition if closeConn is called too quickly, where the channel is created on the observable and closeConn is called at the same time. Also added logic to close the channel immediate if the closeConn was already called.

Checklist

• ./gradlew build compiles code correctly
• Added new tests where applicable
• ./gradlew test passes all tests
• Extended README or added javadocs where applicable

[github-actions]

Test Results

545 tests  ±0   537 ✔️ ±0   7m 39s ⏱️ + 1m 12s
128 suites ±0       8 💤 ±0 
128 files   ±0       0 ❌ ±0 

Results for commit 8ec0a49. ± Comparison against base commit d09cb9a.

♻️ This comment has been updated with latest results.

[github-actions]

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