Docs: Indicate that the Children Aggregation is coming in 1.4.0
[CORE] Notify listener when execution was rejected
Add missing cluster blocks handling for master operations
Master node related operations were missing proper handling of cluster blocks, allowing for example to perform cluster level update settings even before the state was fully restored on initial cluster startup

Note, the change allows to change read only related settings without checking for blocks on update settings, as without it, it means one can't re-enable metadata/write. Also, it doesn't check for blocks on cluster state and health API, as those are allowed to be used even when blocked to figure out what causes the block.
closes #7763
closes #7740
Internal: make sure that internally generated percolate request re-uses the original headers and request context

Closes #7767
Aggregations: More consistent response format for scripted metrics aggregation

Changes the name of the field in the scripted metrics aggregation from 'aggregation' to 'value' to be more in line with the other metrics aggregations like 'avg'