[Task] Add TaskTraits.h and ThenFunctionTraits struct.
[Task] Fix: Task< void >::Then() now resolves then task with ThenFunctionTraits.
- Add RegularTask< Result >.
[Task] Simplify TaskHolder classes.
- Remove ContinuationHolder interface.
- Remove m_host in BasicTask class.
[Task] Add using m_result in RegularTask<R> and ThenTask<R>.
- This is for NDK compiling.
[Task] Change TaskImpl::m_holder to shared_ptr.
[Task] Add Task constructor by holders ( std::shared_ptr< Detail::TaskHolder > ).
(minor) Use Visual Leak Detector in unit test.
[Task] ThenTask get antecedent when creating.