Rollup QSQ follow-up

[petardz]

Issue #, if available:

Description of changes:

To support QueryStringQuery queries we're using core's QueryStringQueryParser which requires us to have mappings of index(QueryShardContext --> MapperService) for which we're parsing query_string. In our case that is sourceIndex and since we want to support this type of query, even if sourceIndex is deleted, we need to store sourceIndex mappings inside rollup job. We can only store mappings for fields that we allow user to search on: all dimensions except DATE_HISTOGRAM field.

1. Added saving of field mappings of sourceIndex fields of interest inside rollup job in sourceIndexFieldMappings field
2. Added populating newly added field sourceIndexFieldMappings during Rollup Create/Update actions
3. Added updating rollup job inside rollup index mapping _meta field from RollupInterceptor, to handle old rollup jobs created before this PR.

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