Recently we introduced the settings cluster.remote to take the place of
search.remote for configuring remote cluster connections. We made this
change due to the fact that we have generalized the remote cluster
infrastructure to also be used within cross-cluster replication and not
only cross-cluster search. For backwards compatibility, when we made this
change, we allowed that cluster.remote would fallback to
search.remote. Alas, the initial change for this contained a bug for
handling the proxy and seeds settings. The bug for the seeds settings
arose because we were manually iterating over the concrete settings only
for cluster.remote seeds but not for search.remote seeds. This commit
addresses this by iterating over both cluster.remote seeds and
search.remote seeds. Additionally, when checking for existence of proxy
settings, we have to not only check cluster.remote proxy settings, but
also fallback to search.remote proxy settings. This commit addresses
both issues, and adds tests for these situations.
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