fdb 730 remotefdb teardown

[mcakircali]

Description

fixes connection lifecycle issue:
under concurrent usage, regarding connection "re-use": there's a gap that while tearing down, another client can try to re-use the connection.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

🌈🌦️📖🚧 Documentation FDB 🚧📖🌦️🌈
https://sites.ecmwf.int/docs/fdb/pull-requests/PR-337

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.14085% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.48%. Comparing base (09312de) to head (b2c4a53).

Files with missing lines	Patch %	Lines
...sts/regressions/FDB-730/fdb_730_regression_test.cc	89.85%	7 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #337      +/-   ##
===========================================
+ Coverage    77.36%   77.48%   +0.11%     
===========================================
  Files          410      411       +1     
  Lines        27483    27553      +70     
  Branches      2758     2766       +8     
===========================================
+ Hits         21263    21350      +87     
+ Misses        6220     6203      -17     

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