fix: improve destroy signal handling

[rustatian]

Reason for This PR

ref: roadrunner-server/roadrunner#1567

Description of Changes

• update destroy contex usage, set it only if it's coming empty.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the MIT license.

PR Checklist

[Author TODO: Meet these criteria.]
[Reviewer TODO: Verify that these criteria are met. Request changes if not]

• All commits in this PR are signed (git commit -s).
• The reason for this PR is clearly provided (issue no. or explanation).
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this PR.
• Any user-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.

[codecov]

Codecov Report

Merging #72 (edf9001) into master (eaaffc5) will decrease coverage by 0.77%.
The diff coverage is 38.23%.

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
- Coverage   83.64%   82.87%   -0.77%     
==========================================
  Files          22       22              
  Lines        1877     1904      +27     
==========================================
+ Hits         1570     1578       +8     
- Misses        252      271      +19     
  Partials       55       55              

Impacted Files	Coverage Δ
pool/static_pool/static_pool.go	71.98% <36.36%> (-5.80%)	⬇️
worker_watcher/worker_watcher.go	89.04% <100.00%> (ø)

... and 1 file with indirect coverage changes