Fix accessing already freed data in extended query.

Fix accessing already freed data in extended query.

When previous_message was set and accessed later on, it looked into
already freed data. This is because read_kind_from_backend() set the
pointer to the previous message in session context, and then released
the memory at the end of the function. No report from fields so far. I
just found this while testing Pgpool-II.

To fix this, store the previous message in the session context, rather
than storing the pointer. Also have a flag in the session context to
indicate whether a previous message is actually stored or not.

All the API for previous message has not been changed.