storage: fix local_call not finding C stored procedures

Currently, if function was created as C stored procedure with
box.schema.func.create, all types of router.call and router.map_callrw
cannot find it and return `Procedure 'name' is not defined` error.
This is cased by the fact that both of these function use local_call,
which invokes net_box.self.call, which doesn't currently work with
C stored procedures due to the bug.

Let's use box.func, where it's possible instead of net_box.self.call
in local_call.

Closes #436

NO_DOC=bugfix
NO_TEST=<already tested>

vshard/storage/init.lua

@@ -251,7 +251,17 @@ end
 -- exceptions are not allowed.
 --
 local function local_call(func_name, args)
-    return pcall(netbox_self_call, netbox_self, func_name, args)
+    -- If function was created with box.schema.func we call it via
+    -- func.call API, as net_box.self.call doesn't work with C stored
+    -- procedures up to Tarantool 3.0. If box.func is inaccessible,
+    -- which is true for all versions below 2.2, we fallback to
+    -- net_box.self.call too.
+    if box.func == nil or box.func[func_name] ==  nil then
+        return pcall(netbox_self_call, netbox_self, func_name, args)
+    end
+
+    local func = box.func[func_name]
+    return pcall(func.call, func, args)
 end
 
 --