Introduce connections_lock to protect Server.connections (backport 1.9)

[Drvi]

Backport of #1161

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 77.77778% with 4 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (release-1.9@f20777d). Click here to learn what that means.

Files	Patch %	Lines
src/Servers.jl	77.77%	4 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##             release-1.9    #1162   +/-   ##
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  Coverage               ?   82.49%           
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  Files                  ?       32           
  Lines                  ?     3056           
  Branches               ?        0           
==============================================
  Hits                   ?     2521           
  Misses                 ?      535           
  Partials               ?        0           

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[Drvi]

I think release-1.9 is missing some fixes by Jameson which makes master pass on nightly (#1146, #1148, #1149)

[Drvi]

@quinnj Would you be ok with merging this backport?

[quinnj]

Sure thing; go ahead