[BUGFIX] FlowSession: Fixed renewId()
This patch provides a new implementation of FlowSession's renewId()
method. It previously loaded all data from the old cache entry in order
to write it into a cache entry with the new session identifier. This
effectively overwrote any existing session-scoped object which existed
in the old cache data.
The new implementation uses a session identifier independent internal
storage identifier which doesn't change on renewing the public session
identifier. This way we don't need to move around session data, but
can simply store a new mapping between session id and storage id.
This patch also lets the HTTP Request store only the baseUri instead of
the whole settings array which leads to a smaller footprint and less
information stored in a session (as the current request is, in most
cases, part of the serialized session data).