Bulk API: Do not fail whole request on closed index

The bulk API request was marked as completely failed,
in case a request with a closed index was referred in
any of the requests inside of a bulk one.

Implementation Note: Currently the implementation is a bit more verbose in order to prevent an `instanceof` check and another cast - if that is fast enough, we could execute that logic only once at the beginning of the loop (thinking this might be a bit overoptimization here).

Closes #6410
maybe we should call it `Indexable` or `DocumentRequest` something like this and it should also have a routing or even better the common denominator of update / delete / index ie all the methods they share?