Make SSLContext's exit() handling more graceful

Make SSLContext's exit() handling more graceful

Summary:
Some platforms that SSLContext run on call exit() despite best practices.
This cause the static structures to be destructed and cause race conditions
and crashes.

This new's the static structures so that they won't be destructed on exit()

Test Plan: Unit tests

Reviewed By: seanc@fb.com

Subscribers: trunkagent, ssl-diffs@, seanc, njormrod, folly-diffs@

FB internal diff: D1702186

Tasks: 5572637

Signature: t1:1702186:1416941649:c5bdfd8cc604fc3618f51bdb8b910b5b2cf350ad