Fix FlowSessionImpl serialization after deserialization

[yurem]

This update fix serialization issue when server need to serialize FlowSessionImpl.java after deserialization. In this flow server throws NPE in line 160 because deserialization not restores flow value. Instead of this it set only flowIdvalue. The issue exists in 2.5.1.RELEASE and 3.0.0-RC1.

This error is easy to reproduce with jetty server and enabled module session-store-jdbc and session-cache-null.