Unify REST responses for Acknowledged[Request|Response]

All Rest handlers that use AcknowledgedResponse look identical. Since
we gonna have more of them in the future we can abstract the logic away.
CompletionFieldMapper: Return error if weight is no integer

In order to make sure that people do not get confused, if they
index a float as weight, it makes more sense to reject it instead of
silently parsing it to an integer and using it.

The CompletionFieldMapper now checks for the type of the number which
is being read and throws and exception if the number is something else
than int or long.

Closes #3977