We want to be able to cancel decommission when it's still in the
tablet draining phase. Such a request is in a pending and paused
state, and can be safely canceled. We set the node's "draining" flag
back to false.
When API user requests status of a virtual task, we first need to find
which virtual_task instance tracks given operation. While doing this we
gather some info regarding the task, but we don't utilize it.
Add virtual_task_hint that keeps info that was gathered during virtual
task lookup and pass it to virtual_task's methods so the info doesn't
need to be retrieved twice.
