In real deployments it is important that clusters are properly configured to
avoid accidentally forming multiple independent clusters at cluster
bootstrapping time. However we also expect to be able to unpack Elasticsearch
and start up one or more nodes without any up-front configuration, and have
them do their best to find each other and form a cluster after a few seconds.
This change adds a delayed automatic bootstrapping process to nodes that start
up with no relevant settings set to support the desired out-of-the-box
experience without compromising safety in properly-configured deployments.
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