Core: Add warmer listener only when index service is set, in order to prevent possible NPE.

The IndicesWarmer gets set before the InternalIndexService gets set, which can lead to a small time window were InternalIndexService isn't set

Closes #8140
Closes #8168
Core: Forget to move the child filter over to fixed bitset filter.

Closes #8171
Aggregations: the `children` agg didn't take deleted document into account.

The live docs that is passed down was ignored by the filter impl. Now the children filter gets wrapped with ApplyAcceptedDocsFilter, so live docs are actually applied.

Closes #8180