More complete OpenSslEngine SSLSession implementation

Motivation:

The current SSLSession implementation used by OpenSslEngine does not support various operations and so may not be a good replacement by the SSLEngine provided by the JDK implementation.

Modifications:

- Add SSLSession.getCreationTime()
- Add SSLSession.getLastAccessedTime()
- Add SSLSession.putValue(...), getValue(...), removeValue(...), getValueNames()
- Add correct SSLSession.getProtocol()
- Ensure OpenSSLEngine.getSession() is thread-safe
- Use optimized AtomicIntegerFieldUpdater when possible

Result:

More complete OpenSslEngine SSLSession implementation