Move OpenSSL uninitialization to a separate function

Move OpenSSL uninitialization to a separate function

Summary:
The current behaviour of SSLContext is subject to a bug where an instance going
out of scope at the wrong time can decrement the global refcount to 0, causing
the destructor to destroy the SSL context. Instead, this requires callers to
explicitly uninitialize the SSLContext instance and assume the risk of wrongly
destroying the global SSL context.

Test Plan:
Tested with mcrouter

Reviewed By: subodh@fb.com

Subscribers: afrind, ssl-diffs@, folly-diffs@, yfeldblum, chalfant, dihde, melitam

FB internal diff: D1949649

Tasks: 6358211

Signature: t1:1949649:1427854689:aea2dc801f63256ff64188b0f7a15121dcecee69