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 
The CrashStorage used to have a caching mechanism here to avoid trying to create an index every time we process a crash report. I think it's a severe regression to remove it, since it adds one HTTP request per processed crash, which seems to be quite heavy to me. Can you pull that in from the previous module?