MINOR: counters: make it easier to extend the amount of tracked counters

By properly affecting the flags and values, it becomes easier to add
more tracked counters, for example for experimentation. It also slightly
reduces the code and the number of tests. No counters were added with
this patch.
diff --git a/include/types/session.h b/include/types/session.h
index 7b52182..b5c7460 100644
--- a/include/types/session.h
+++ b/include/types/session.h
@@ -86,10 +86,13 @@
 #define	SN_FINST_SHIFT	16		/* bit shift */
 
 #define SN_IGNORE_PRST	0x00080000	/* ignore persistence */
-#define SN_BE_TRACK_SC1 0x00100000	/* backend tracks stick-counter 1 */
-#define SN_BE_TRACK_SC2 0x00200000	/* backend tracks stick-counter 2 */
+
+#define SN_COMP_READY   0x00100000	/* the compression is initialized */
 
-#define SN_COMP_READY   0x00400000	/* the compression is initialized */
+/* session tracking flags: these ones must absolutely be contiguous */
+#define SN_BE_TRACK_SC1 0x00200000	/* backend tracks stick-counter 1 */
+#define SN_BE_TRACK_SC2 0x00400000	/* backend tracks stick-counter 2 */
+#define SN_BE_TRACK_ANY 0x00600000      /* union of all SN_BE_TRACK_* above */
 
 
 /* WARNING: if new fields are added, they must be initialized in event_accept()