BUG/MINOR: session: atomically increment the tracked sessions counter

In session_count_new() the tracked counter was still incremented with
a "++" outside of any lock, resulting in occasional slightly off values
such as the following:

    # table: foo, type: string, size:1000, used:1
    0xb2a398: key=127.1.2.3 use=0 exp=86398318 sess_cnt=999959 http_req_cnt=1000004

Now with the correct atomic increment:

    # table: foo, type: string, size:1000, used:1
    0x7f82a4026d38: key=127.1.2.3 use=0 exp=86399294 sess_cnt=1000004 http_req_cnt=1000004

This can be backported to 1.8.

src/session.c

@@ -118,7 +118,7 @@ static void session_count_new(struct session *sess)
 
 		ptr = stktable_data_ptr(stkctr->table, stkctr_entry(stkctr), STKTABLE_DT_SESS_CNT);
 		if (ptr)
-			stktable_data_cast(ptr, sess_cnt)++;
+			HA_ATOMIC_ADD(&stktable_data_cast(ptr, sess_cnt), 1);
 
 		ptr = stktable_data_ptr(stkctr->table, stkctr_entry(stkctr), STKTABLE_DT_SESS_RATE);
 		if (ptr)