async_at_hpt needs a more accurate name

[DaoWen]

Following the comment in hclib-async.hpp and the related discussion in #52, async_at_hpt should be renamed to something less confusing.

The "at_hpt" suffix makes it sound like it takes an HPT argument rather than a place. Using async_at_place might be a better option.

[budimlic]

Do we have "async_at" anything other than a place? I would prefer simply "async_at".