Asynchronous writing to tags storage

[kirillt]

At the moment, after tags are changed for a resource, we wait till new state of tags storage is written into filesystem. We can update the storage file in background allowing user to continue working. Right now there is slight delay less than 0.5 sec after applying new tags which can be removed. We must ensure that such coroutines complete even in case of a crash, because we can't afford to lose user's data. So this features depends on #173.