[stable-2.21] BugFix - DNSCache Lock

[backportbot]

Backport of PR #1770

[github-actions]

SpotBugs

Category	Base	New
Bad practice	34	34
Correctness	34	34
Dodgy code	26	26
Internationalization	6	6
Malicious code vulnerability	49	49
Multithreaded correctness	11	11
Performance	8	8
Total	168	168

[codecov]

Codecov Report

Attention: Patch coverage is 30.76923% with 9 lines in your changes missing coverage. Please review.

Project coverage is 43.06%. Comparing base (70da635) to head (9ca24b9).
Report is 4 commits behind head on stable-2.21.

Files with missing lines	Patch %	Lines
...ary/src/main/java/com/nextcloud/common/DNSCache.kt	30.76%	9 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                @@
##             stable-2.21    #1772      +/-   ##
=================================================
- Coverage          43.07%   43.06%   -0.02%     
- Complexity           987      989       +2     
=================================================
  Files                235      235              
  Lines               8566     8566              
  Branches            1122     1123       +1     
=================================================
- Hits                3690     3689       -1     
- Misses              4359     4362       +3     
+ Partials             517      515       -2     

Files with missing lines	Coverage Δ
...ary/src/main/java/com/nextcloud/common/DNSCache.kt	43.75% <30.76%> (-8.34%)	⬇️

... and 2 files with indirect coverage changes

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