librados/asio: add version_t to completion signatures

librados/asio: add version_t to completion signatures

IoCtx::aio_operate() doesn't update IoCtx::get_last_version(). to make
the resulting version_t available to the caller, we have to read it out
of the AioCompletionImpl and return it to the caller

Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit 5cc7cf44dba51f8071a6b7cd919c019283ac4ce1)