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 
(Glad I caught this) Using ``create`` here might be a source of problems. ``create`` means that if the document already exists, the insertion will fail. Yet that is not what we want: we regularly re-process crash reports, re-indexing them in our databases. When that happens, we want the previous crash to be replaced with the new one. I thus think we want to use ``index`` here instead of ``create``.