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.
Currently, lookup_virtual_task gets the list of ids of all operations
tracked by a virtual task and checks whether it contains given id.
The list of all ids isn't required and the check whether one particular
operation id is tracked by the virtual task may be quicker than listing
all operations.
Add virtual_task::contains method and use it in lookup_virtual_task.
