Add time stamps to index names in scripts

Combined with Elasticsearch index aliases, this should fix #94:
when reindexing, a new index is created. When done, we can update
the aliases used by the API to point to the new indexes, and
finally delete the old indexes.
Some performance tweaks & cleanup in collection and conversion jobs

Only look up subjects in the map file for triples with specified
predicates. Some minor tweaks and cleanup (pull things up from map
functions into mapper setup to do them only once).