Update exposure event tracking and add docs

[efahk]

Submitting the call to track an exposure event on a threadpool was deadlocking in eventlet when threading module is patched.

For now, for more predictability across runtimes, updating calling track directly. Clients can override the consumer with more complex send behavior.
Other updates to use a single thread for polling, adding some public method comments, & some test refactors

[codecov]

Codecov Report

❌ Patch coverage is 98.76543% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.13%. Comparing base (0f8c84e) to head (d466228).

Files with missing lines	Patch %	Lines
mixpanel/flags/local_feature_flags.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
+ Coverage   92.45%   93.13%   +0.68%     
==========================================
  Files           8        8              
  Lines        1153     1151       -2     
  Branches       76       76              
==========================================
+ Hits         1066     1072       +6     
+ Misses         56       49       -7     
+ Partials       31       30       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.