Create hpx::get_ptr<>() allowing to access component implementation

[hkaiser]

Add an API function

template <typename Component>
boost::shared_ptr<Component> get_ptr(id_type);

which returns a pointer to the underlying component instance represented by the given id. The returned shared_ptr needs to perform several implicit operations:

• hold the referenced component alive
• 'lock' the component instance, preventing it from being migrated.

This function will return a valid pointer only if the component instance is currently located on the requesting locality. It should report an error otherwise.

[hkaiser]

This has now changed to the following interface:

template <typename Component>
hpx::future<boost::shared_ptr<Component> > hpx::get_ptr(hpx::naming::id_type const& id);