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 
I don't think we care about those two checks, and they make the test less robust. Those are internal behaviour that we don't really care about, as long as the crash is sent to Elasticsearch.

Think about it this way: if you change the ``ESCrashStorage`` class so that it does one less call to ``Elasticsearch``, but still sends the crash to ES as expected, then your test will fail even though it should not.