avoid memory leaks if linked with static openssl libs (#1857)

* New function SSLServer::update_certs. Allows to update certificates while server is running

* New function SSLServer::update_certs. Added unit test

* avoid memory leaks if linked with static openssl libs

---------

Co-authored-by: CEU\schielke <Rainer.Schielke@heidelberg.com>

httplib.h

@@ -726,6 +726,10 @@ private:
         assert(true == static_cast<bool>(fn));
         fn();
       }
+
+#ifdef CPPHTTPLIB_OPENSSL_SUPPORT
+      OPENSSL_thread_stop ();
+#endif
     }
 
     ThreadPool &pool_;