Stateless session is not serializable

[fredericDelaporte]

Stateless session is not serializable, although flagged as such: at least, it cannot be deserialized.

• It is not restoring its connection manager, which can only lead to null reference exceptions. (May change with NH-3606 - open a stateless session from a session #1520.)
• It is not restoring its temporaryPersistenceContext, which will also lead to null reference exceptions.
• It has no suitable constructor for allowing deserialization. (Binary serialization does not call any in its basic pattern, so this is not an issue.)
• It should check some preconditions before serializing, as do SessionImpl.

But well, what is the point of serializing a stateless session? Better close and open a new one I think. So set as trivial.

[hazzik]

There are 2 options: either we mark it as not serializable, or implement proper serialization despite the motivation of why anyone would want to serialize it.