librados: release pimpl pointer in destructor

librados: release pimpl pointer in destructor

before this change, the librados applications are responsible to call
`AioCompletion::release()` explicitly to release its internal pimpl
pointer. this is error prone and not intuitive.

after this change, the destructor of `AioCompletion` and
`PoolAsyncCompletion` will do this automatically. while
`AioCompletion::release()` and `PoolAsyncCompletion::release()` still
delete the instance as they did before. so this change is backward
compatible, as existing librados clients can still use `ptr->release()`
to free the completion instance, while new clients can just `delete
ptr`.

librados_test_stub is updated accordingly to match the new model

Signed-off-by: Kefu Chai <kchai@redhat.com>