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 
How about adding a test that succeeds after an operational error? That would be same as this one, but the side effect logic would use a counter and stop raising the exception after a few tries. That seems to me like an important behaviour to test.