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 
Ah, yes, good call.  As we discussed on IRC, here's a [feature proposal](https://bugzilla.mozilla.org/show_bug.cgi?id=1113583) for this mechanism (when we're out of feature-freeze).