librados: deprecate aio_operate() read variant that takes snapid

The argument was ignored.

Signed-off-by: Sage Weil <sage@inktank.com>
librados: mark old get_version() as deprecated

Use the newly-discovered (for me) deprecated attribute to mark the old
get_version() method and point users toward get_version64().  And fix a
couple of users in the kvstore code!

Signed-off-by: Sage Weil <sage@inktank.com>