* Handle master failure in NodeSeenService

NodeSeenService can miss seeing nodes if the master changes while it's
processing the cluster state update which adds the nodes to the cluster.
This caused occasional test failures in the test intended to check that
NodeSeenService is working as intended.

This commit adjusts NodeSeenService's early returns to ensure that, if
the master changed, the new master checks for seen nodes even if nodes
were not added in that particular cluster state update.

* Clarify & correct "just-became-master" check

* Remove leftover debug logging (d'oh!)

Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>