[MEDIUM] session: add a counter on the cumulated number of sessions

Sessions are like connections but they have been accepted by L4 rules
and really became sessions.
diff --git a/src/stick_table.c b/src/stick_table.c
index d701d09..e68cae3 100644
--- a/src/stick_table.c
+++ b/src/stick_table.c
@@ -528,6 +528,7 @@
 	[STKTABLE_DT_SERVER_ID] = { .name = "server_id", .data_length = stktable_data_size(server_id) },
 	[STKTABLE_DT_CONN_CNT]  = { .name = "conn_cnt",  .data_length = stktable_data_size(conn_cnt)  },
 	[STKTABLE_DT_CONN_CUR]  = { .name = "conn_cur",  .data_length = stktable_data_size(conn_cur)  },
+	[STKTABLE_DT_SESS_CNT]  = { .name = "sess_cnt",  .data_length = stktable_data_size(sess_cnt)  },
 	[STKTABLE_DT_BYTES_IN_CNT]  = { .name = "bytes_in_cnt",  .data_length = stktable_data_size(bytes_in_cnt)  },
 	[STKTABLE_DT_BYTES_OUT_CNT] = { .name = "bytes_out_cnt", .data_length = stktable_data_size(bytes_out_cnt) },
 };