Not load the ids of child documents into memory.
Closes #3028
Expose LimitTokenCountFilter as a TokenFilter

Closes #3013
Added test for LimitTokenCountFilterFactory
Allow to disable allocation on the index level
Similar to the global cluster wide disable allocation flags, allow to set those on a specific index by updating its settings. The keys are the same as the cluster one, except they start with an index, for example: index.routing.allocation.disable_allocation set to true.
closes #3031
Make sure only relevant documents are evaluated in the second round lookup phase.

Both has_parent and has_child filters are internally executed in two rounds. In the second round all documents are evaluated whilst only specific documents need to be checked. In the has_child case only documents belonging to a specific parent type need to be checked and in the has_parent case only child documents need to be checked.

Closes #3034