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
is there a reason you made setSessionCacheEnabled() return a boolean? we used to return 'this' or 'void'