librados: Make ObjectOperation move constructible

librados: Make ObjectOperation move constructible

Since it just holds a pointer to a heap-allocated ObjectOperationImpl,
there's no reason not to. This allows us to move them around so the
call to aio_operate can happen after the stack frame where the
ObjectOperation was created has exited.

Would have just switched to a unique_ptr but the test harness relies
on being able to slip its own stuff in there and it was less work to
write my own move constructors than edit all the casts there to use
the get method.

Since there is no move construction now, no code will be broken if we
assert on use of moved-from objects and feedback suggests that people
think this is likelier to catch erroneous use than lazy
reinitialization.

Signed-off-by: Adam C. Emerson <aemerson@redhat.com>