Wait jaeger receiver server goroutines exit on shutdown

[pjanotti]

The test hitting #2975 runs Start and Shutdown in close succession and in this case Shutdown was setting the field collectorServer to nil before the goroutine launched on Start actually accessed the field. Fixed the issue by adding a sync.WaitGroup to ensure that Shutdown actually waits for all launched goroutines to exit (also improved error handling for the server goroutines). Removed setting the fields to nil on Shutdown since that is not necessary.

Fixes #2975

[codecov]

Codecov Report

Merging #2985 (db73e4c) into main (d10b842) will decrease coverage by 0.00%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##             main    #2985      +/-   ##
==========================================
- Coverage   91.65%   91.64%   -0.01%     
==========================================
  Files         312      312              
  Lines       15427    15435       +8     
==========================================
+ Hits        14139    14145       +6     
- Misses        881      882       +1     
- Partials      407      408       +1     

Impacted Files	Coverage Δ
receiver/jaegerreceiver/trace_receiver.go	73.55% <75.00%> (+0.05%)	⬆️

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