Explicit allow to enable / disable session cache

Motivation:

It is sometimes useful to enable / disable the session cache.

Modifications:

* Add OpenSslSessionContext.setSessionCacheEnabled(...) and isSessionCacheEnabled()

Result:

It is now possible to enable / disable cache on the fly
@trustin only because the JNI calls does. But I'm fine with whatever you prefer. Just let me know... maybe void ?