fix: shut down "session flusher" more promptly

[bukzor]

Currently this thread will wait for sleep(self.flush_interval) to end before
checking self._running. We can do better by using an Event.

Now it will wait for self.flush_interval or until self.__shutdown_requested
is set, whichever is shorter.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.72%. Comparing base (a7b2d67) to head (81cf1a2).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
sentry_sdk/sessions.py	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4561      +/-   ##
==========================================
+ Coverage   80.68%   80.72%   +0.04%     
==========================================
  Files         156      156              
  Lines       16482    16481       -1     
  Branches     2802     2801       -1     
==========================================
+ Hits        13299    13305       +6     
+ Misses       2299     2297       -2     
+ Partials      884      879       -5     

Files with missing lines	Coverage Δ
sentry_sdk/sessions.py	79.72% <87.50%> (+0.55%)	⬆️

... and 3 files with indirect coverage changes

[sl0thentr0py]

ignore previous review, was confused by the negation