[MINOR] http: move appsession 'sessid' from session to http_txn

This change, suggested by Cyril Bonté, makes a lot of sense and
would have made it obvious that sessid was not properly initialized
while switching to keep-alive. The code is now cleaner.
diff --git a/src/client.c b/src/client.c
index 866fc87..b0184bb 100644
--- a/src/client.c
+++ b/src/client.c
@@ -185,7 +185,6 @@
 		s->be = s->fe = p;
 
 		s->req = s->rep = NULL; /* will be allocated later */
-		s->sessid = NULL;
 
 		s->si[0].state = s->si[0].prev_state = SI_ST_EST;
 		s->si[0].err_type = SI_ET_NONE;
@@ -263,6 +262,7 @@
 		 * session.c:session_free(). It is important that they are
 		 * properly initialized.
 		 */
+		txn->sessid = NULL;
 		txn->srv_cookie = NULL;
 		txn->cli_cookie = NULL;
 		txn->uri = NULL;