This change removes the InternalClient and the InternalSecurityClient. These are replaced with
usage of the ThreadContext and a transient value, `action.origin`, to indicate which component the
request came from. The security code has been updated to look for this value and ensure the
request is executed as the proper user. This work comes from elastic/x-pack-elasticsearch#2808 where @s1monw suggested
that we do this.
While working on this, I came across index template registries and rather than updating them to use
the new method, I replaced the ML one with the template upgrade framework so that we could
remove this template registry. The watcher template registry is still needed as the template must be
updated for rolling upgrades to work (see elastic/x-pack-elasticsearch#2950).
Original commit: elastic/x-pack-elasticsearch@7dbf2f263e
0a683a0e18