Ignore live docs when loading field data, the ID cache and filter caches.

Relying on deleted documents when loading field data is dangerous because a
field data instance might be loaded for a given generation of a segment and
then loaded from the cache by an older generation of the same segment which
has fewer deleted documents. This could, for example, lead to under-estimated
facet counts. The same issue applies to the ID cache and filter caches.

Close #3224
Fixes issue that in some cases the doc_as_upsert option is ignored.
Closes #3265
Changes validation error message
Clean up update tests
Stop aborting of multiget requests in case of missing index

The MultiGet API stops with a IndexMissingException, if only one of all
requests tries to access a non existing index. This patch creates a
failure for this item without failing the whole request.

Closes #3267
Supports mget fields parameter given as string.

Closes 3270
Parent was ignored in exists request

The routing was always set to null right after the parent was set.

Closes #3276