BUG/MINOR: cache: fix cache tree iteration

Ever since the introduction of multiple cache trees, the "show cache"
CLI command was not properly showing the contents of each tree, but was
only showing the first one.
Fix that by properly resetting next_key when we switch to the next tree.

Should be backported up to 3.0.

src/cache.c

@@ -3047,6 +3047,7 @@ static int cli_io_handler_show_cache(struct appctx *appctx)
 				node = eb32_lookup_ge(&cache_tree->entries, next_key);
 				if (!node) {
 					ctx->next_key = 0;
+					next_key = 0;
 					break;
 				}