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 
As we discussed on IRC, this appears to be related to the previous lib's default formatting choices (i.e. the `T`), and we may need to massage incoming documents before indexing them; either that, or change the mapping (which comes with its own problems).