Make Sentry.{close,get_main_hub} thread-safe

[solnic]

Since we're dealing with a bunch of global vars in Sentry.close, it should be wrapped in a mutex sync because threads may be accessing these vars while Sentry is closing, and THAT probably causes #2386 and #2396.

#skip-changelog

[solnic]

Sentry.initialized? should use a mutex too, given that it relies on @main_hub that may get reset to nil by Sentry.close

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.17%. Comparing base (b4bbed2) to head (25454b2).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2436      +/-   ##
==========================================
- Coverage   98.34%   98.17%   -0.17%     
==========================================
  Files         126      126              
  Lines        4703     4725      +22     
==========================================
+ Hits         4625     4639      +14     
- Misses         78       86       +8     

Components	Coverage Δ
sentry-ruby	98.51% <100.00%> (-0.14%)	⬇️
sentry-rails	97.19% <ø> (-0.76%)	⬇️
sentry-sidekiq	97.47% <ø> (+1.51%)	⬆️
sentry-resque	92.85% <ø> (+0.21%)	⬆️
sentry-delayed_job	95.65% <ø> (ø)
sentry-opentelemetry	99.31% <ø> (-0.69%)	⬇️

Files with missing lines	Coverage Δ
sentry-ruby/lib/sentry-ruby.rb	99.54% <100.00%> (+<0.01%)	⬆️

... and 17 files with indirect coverage changes