packaged_task::reset should not reuse the shared state to conform to C++11

[viboes]

The standard says

void reset();
Effects: as if *this = packaged_task(std::move(f)), where f is the task stored in *this. [ Note: This constructs a new shared state for *this. The old state is abandoned (30.6.4). — end note ]
Throws:
— bad_alloc if memory for the new shared state could not be allocated.
— any exception thrown by the move constructor of the task stored in the shared state. 
— future_error with an error condition of no_state if *this has no shared state.

Boost.Thread reuse the shared state.

Effects: Reset the state of the packaged_task so that it can be called again.