MEDIUM: stick-table: implement lookup from a sample fetch

Currently we have stktable_fetch_key() which fetches a sample according
to an expression and returns a stick table key, but we also need a function
which does only the second half of it from a known sample. So let's cut the
function in two and introduce smp_to_stkey() to perform this lookup. The
first function was adapted to make use of it in order to avoid code
duplication.
diff --git a/include/proto/stick_table.h b/include/proto/stick_table.h
index 57ca223..a121ad5 100644
--- a/include/proto/stick_table.h
+++ b/include/proto/stick_table.h
@@ -46,6 +46,7 @@
 struct stksess *stktable_lookup(struct stktable *t, struct stksess *ts);
 struct stksess *stktable_lookup_key(struct stktable *t, struct stktable_key *key);
 struct stksess *stktable_update_key(struct stktable *table, struct stktable_key *key);
+struct stktable_key *smp_to_stkey(struct sample *smp, struct stktable *t);
 struct stktable_key *stktable_fetch_key(struct stktable *t, struct proxy *px,
 				        struct session *l4, void *l7, unsigned int opt,
 				        struct sample_expr *expr, struct sample *smp);