Allow to obtain session as byte[]

Motivation:

Often it is useful to be able to get a session as a byte[] representation for later re-use or storage.

Modifications:

Add SSL.getSessionId(...) which allows to retrive a byte[] representation of the session.

Result:

It's now possible to store a session for later usage.
this needs an update. We don't use `i2d_SSL_SESSION` anymore.