librados: drop reference to completion in container destructor

Move the PoolAsyncCompletionImpl reference drop from
C_PoolAsync_Safe::finish() to ~C_PoolAsyncSafe(), as finish() is only
called when the async request is actually sent.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
librados: add some clarifying comments

about async pool operation handle

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge branch 'wip-6444' into next

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>