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 
Let's just rename this function to ``indices_client`` or ``index_client`` or ``index_connection`` (or anything, the only thing that bothers me is the lack of an underscore in the name :P ).