The retention lease syncs need to occur under the system context,
because they are internal actions executed on behalf of the user. Today
we are relying on this happening for background syncs by virtue of the
fact that the context the syncs are created under is the system
context. This is due to these occurring on the cluster state applier
thread. However, there are situations where this does not hold such as
when a timed out cluster state publication occurs, and the node where
the shard is allocated is the elected master node. In that case, the
context will be empty due to the fact that we do not reschedule
publication under the system context. Currently, doing so runs us into
some troubles with losing the existing context, possibly dropping
deprecation headers. We could copy that context over when marking the
current context as the system context, but the implications of that
require some more investigation. For now, we explicitly mark the
retention lease syncs as executing under the system context, as this is
situation that we can reason about.
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