Taken tasks is released back to 'ready' state after reload

Example:
- Worker takes a task
- Run `package.reload()`
- [Task is released back to ready state](https://github.com/tarantool/queue/blob/cede9456e27ed695e865274b44768d5416e6e128/queue/abstract.lua#L581).
- Worker cannot ack task

https://github.com/tarantool/queue/issues/66
https://github.com/tarantool/queue/pull/106