Fix panic when dealing with crashing discoveries

[cmaglie]

Previously, a pluggable discovery that crashed at startup would cause the client library to panic.

goroutine 11 [running]:
testing.tRunner.func1.2({0x5eaae0, 0x7db1f0})
	/home/cmaglie/Software/go1.22.3/src/testing/testing.go:1631 +0x24a
testing.tRunner.func1()
	/home/cmaglie/Software/go1.22.3/src/testing/testing.go:1634 +0x377
panic({0x5eaae0?, 0x7db1f0?})
	/home/cmaglie/Software/go1.22.3/src/runtime/panic.go:770 +0x132
github.com/arduino/pluggable-discovery-protocol-handler/v2.(*Client).Run(0xc0000fa090)
	/home/cmaglie/Workspace/pluggable-discovery-protocol-handler/client.go:281 +0x16e
github.com/arduino/pluggable-discovery-protocol-handler/v2.TestClient.func1(0xc0000b31e0)
	/home/cmaglie/Workspace/pluggable-discovery-protocol-handler/client_test.go:108 +0xcc
testing.tRunner(0xc0000b31e0, 0x64c850)
	/home/cmaglie/Software/go1.22.3/src/testing/testing.go:1689 +0xfb
created by testing.(*T).Run in goroutine 9
	/home/cmaglie/Software/go1.22.3/src/testing/testing.go:1742 +0x390
exit status 2

This PR fixes this problem and cleans up the mutex usage in the library.

Related to arduino/arduino-cli#2665.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 32.79%. Comparing base (c563e77) to head (cc38790).
Report is 4 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #50       +/-   ##
===========================================
+ Coverage   16.84%   32.79%   +15.95%     
===========================================
  Files           4        4               
  Lines         374      372        -2     
===========================================
+ Hits           63      122       +59     
+ Misses        294      228       -66     
- Partials       17       22        +5     

Flag	Coverage Δ
unit	32.79% <100.00%> (+15.95%)	⬆️

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