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 
Can we not move that part to the ``Connection`` class while we're at it? It looks weird to me to create the actual connection here instead of passing the config and letting the ``Connection`` class create it itself.