REORG: use the name "sample" instead of "pattern" to designate extracted data

This is mainly a massive renaming in the code to get it in line with the
calling convention. Next patch will rename a few files to complete this
operation.
diff --git a/include/types/proxy.h b/include/types/proxy.h
index aa6cec8..e18bb7a 100644
--- a/include/types/proxy.h
+++ b/include/types/proxy.h
@@ -371,7 +371,7 @@
 struct sticking_rule {
 	struct list list;                       /* list linked to from the proxy */
 	struct acl_cond *cond;                  /* acl condition to meet */
-	struct pattern_expr *expr;              /* fetch expr to fetch key */
+	struct sample_expr *expr;               /* fetch expr to fetch key */
 	int flags;                              /* STK_* */
 	union {
 		struct stktable *t;	        /* target table */