MINOR: session: make the number of stick counter entries more configurable

In preparation of more flexibility in the stick counters, make their
number configurable. It still defaults to 3 which is the minimum
accepted value. Changing the value alone is not sufficient to get
more counters, some bitfields still need to be updated and the TCP
actions need to be updated as well, but this update tries to be
easier, which is nice for experimentation purposes.
diff --git a/include/common/defaults.h b/include/common/defaults.h
index 5000d9c..874d2e2 100644
--- a/include/common/defaults.h
+++ b/include/common/defaults.h
@@ -73,6 +73,13 @@
 #define MAX_HDR_HISTORY 10
 #endif
 
+// max # of stick counters per session (at least 3 for sc0..sc2)
+// Some changes are needed in TCP_ACT_TRK_SC* and SN_BE_TRACK_SC* if more
+// values are required.
+#ifndef MAX_SESS_STKCTR
+#define MAX_SESS_STKCTR 3
+#endif
+
 // max # of loops we can perform around a read() which succeeds.
 // It's very frequent that the system returns a few TCP segments at a time.
 #ifndef MAX_READ_POLL_LOOPS