rename build.{BlockDelay=>BlockDelaySecs}.

Since this global is not typed as a time.Duration,
rather as an int, it makes sense to clarify the unit.

node/impl/common/common.go

@@ -122,7 +122,7 @@ func (a *CommonAPI) Version(context.Context) (api.Version, error) {
 		Version:    build.UserVersion(),
 		APIVersion: build.APIVersion,
 
-		BlockDelay: uint64(build.BlockDelay),
+		BlockDelay: uint64(build.BlockDelaySecs),
 	}, nil
 }