bug 1094767; crashstorage type for new ES module

This PR introduces a crashstorage type for the new ES module.  It also modifies the `ConnectionContext` class to bring it in line with the patterns established in the other `external` modules.  The changes to the context required some other modifications to be made to the index creation class (and associated tests).

`r?` @AdrianGaudebert @twobraids 
s/ES/Elasticsearch. It's better to have explicit logs, and I for one always search the logs for the words ``ERROR`` and ``Elasticsearch``.