Add mutex sync to SessionFlusher aggregates

[sl0thentr0py]

fixes #2468

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.17%. Comparing base (a9b3687) to head (1a32a15).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2469   +/-   ##
=======================================
  Coverage   98.17%   98.17%           
=======================================
  Files         128      128           
  Lines        4825     4829    +4     
=======================================
+ Hits         4737     4741    +4     
  Misses         88       88           

Components	Coverage Δ
sentry-ruby	98.57% <100.00%> (+<0.01%)	⬆️
sentry-rails	97.08% <ø> (ø)
sentry-sidekiq	96.96% <ø> (ø)
sentry-resque	92.85% <ø> (ø)
sentry-delayed_job	95.65% <ø> (ø)
sentry-opentelemetry	99.31% <ø> (ø)

Files with missing lines	Coverage Δ
sentry-ruby/lib/sentry/session_flusher.rb	100.00% <100.00%> (ø)