[MINOR] session-counters: use "track-sc{1,2}" instead of "track-{fe,be}-counters"

The assumption that there was a 1:1 relation between tracked counters and
the frontend/backend role was wrong. It is perfectly possible to track the
track-fe-counters from the backend and the track-be-counters from the
frontend. Thus, in order to reduce confusion, let's remove this useless
{fe,be} reference and simply use {1,2} instead. The keywords have also been
renamed in order to limit confusion. The ACL rule action now becomes
"track-sc{1,2}". The ACLs are now "sc{1,2}_*" instead of "trk{fe,be}_*".

That means that we can reasonably document "sc1" and "sc2" (sticky counters
1 and 2) as sort of patterns that are available during the whole session's
life and use them just like any other pattern.
diff --git a/include/types/proto_tcp.h b/include/types/proto_tcp.h
index bee52aa..0ee2ec7 100644
--- a/include/types/proto_tcp.h
+++ b/include/types/proto_tcp.h
@@ -32,8 +32,8 @@
 enum {
 	TCP_ACT_ACCEPT = 1,
 	TCP_ACT_REJECT = 2,
-	TCP_ACT_TRK_FE_CTR = 3,
-	TCP_ACT_TRK_BE_CTR = 4,
+	TCP_ACT_TRK_SC1 = 3,
+	TCP_ACT_TRK_SC2 = 4,
 };
 
 struct tcp_rule {