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 
There's already an `ElasticSearchCrashStorage` in the module that we're attempting to deprecate.  IMHO it's confusing to have two crashstorage classes with the same name.  We can rename it later if we want, I suppose.