Fixes bug 1013322 - Generate the elasticsearch mapping from the master list of fields.

@peterbe r?

Note that this feature is turned off by default through configuration, using the ``use_mapping_file`` option in ``ElasticsearchCrashStorage``. I want to turn it on on stage first and make sure the new mapping way doesn't break indexing. After a week or two of successful indexing, we will turn it on on production. 
Since it's impossible to find a definite answer to this, I will move it to a config option. This way we can test several values on stage and prod and see what can be a good compromise between safety and performance.