If security is enabled today with ccr then the follow index api will
fail with the fact that system user does not have privileges to use
the shard changes api. The reason that system user is used is because
the persistent tasks that keep the shards in sync runs in the background
and the user that invokes the follow index api only start those background
processes.
I think it is better that the system user isn't used by the persistent
tasks that keep shards in sync, but rather runs as the same user that
invoked the follow index api and use the permissions that that user has.
This is what this PR does, and this is done by keeping track of
security headers inside the persistent task (similar to how rollup does this).
This PR also adds a cluster ccr priviledge that allows a user to follow
or unfollow an index. Finally if a user that wants to follow an index,
it needs to have read and monitor privileges on the leader index and
monitor and write privileges on the follow index.
Martijn van Groningen