MINOR: stream: Expose session terminate state via a new sample fetch

MINOR: stream: Expose session terminate state via a new sample fetch

It is now possible to retrieve the session terminate state, using
"txn.sess_term_state". The sample fetch returns the 2-character session
termation state.

Of course, the result of this sample fetch is volatile. It is subject to
change. It is also most of time useless because no termation state is set
except at the end. It should only be useful in http-after-response rule
sets. It may also be used to customize the logs using a log-format
directive.

This patch should fix the issue #2221.