Task manager tasks should be created with make_task method since
it properly sets information about child-parent relationship
between tasks. Though, sometimes we may want to keep additional
task data in classes inheriting from task_manager::task::impl.
Doing it with existing make_task method makes it impossible since
implementation objects are created internally.
The commit adds a new make_task that allows to provide a task
implementation pointer created by caller. All the fields except
for the one connected with children and parent should be set before.
6494de9bb0