[TASK] Rename Node variables

[sfroemkenjw]

What should be done in the scope of this task?

With patch https://github.com/TYPO3-Solr/ext-solr/pull/3665 we migrated Node class to Endpoint. Maybe it would be cool to migrate the PHP variables, too:

leftNode to leftEndpoint rightNode to rightEndpoint

As next step we should check, if it is possible (make sense) to adapt the method names, too:

getSolrConnectionForNodes to getSolrConnectionForEndpoint