Allow to explicit set and get the session_cache_mode

Motivation:

Sometimes it is useful to be able to explicit set the session_cache_mode or get the current used mode.

Modifications:

* add SSLContext.setSessionCacheMode(...) and SSLContext.getSessionCacheMode(...)
* allow to pass 0 into SSLContext.setSessionCacheSize(...) to allow unlimited size

Result:

It's now possible to set the cache mode on the fly.