This commit adds support for system data streams and also the first use
of a system data stream with the fleet action results data stream. A
system data stream is one that is used to store system data that users
should not interact with directly. Elasticsearch will manage these data
streams. REST API access is available for external system data streams
so that other stack components can store system data within a system
data stream. System data streams will not use the system index read and
write threadpools.
a7dbb31765