librados: implement aio_stat

Implement aio stat and also export this functionality to the C API.

Signed-off-by: Filippos Giannakos <philipgian@grnet.gr>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
librados: add aio stat tests

Implement simple write-stat test, and a write-stat-remove-stat test cycle.

Signed-off-by: Filippos Giannakos <philipgian@grnet.gr>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>