Asset <-> Task synchronization

[Katharsas]

Updating of tasks when assets change is quite messy right now because of async asset import.
When asset services detect asset changes and try to update tasks, they assume that tasks are synced with previous asset status (-> tasks have asset properties for previous assets).
But, since asset import is async, they may not be in sync with status quo (for example asset properties of a task may be null, even though matching asset exists, because asset is still being imported asynchronously).

To fix, make make sure user cannot change assets during running import and make sure SVN changes are put into a queue (instead of causing import / task updates), where all changes add up, until asset import is done. Then run import / task updates in one pass.