Use sqlalchemy yield_per method for fetch rows from huge tables.

Without this fix nailgund need enormous amounts memory. With this fix JSONHandler.fetch_collection don't fetch all rows from database to memory and fetch maximum YIELD_PER rows during iteration over all rows. YIELD_PER parameter may be defined in settings.yaml, in other ways YIELD_PER = 100 by default.

JSONHandler.fetch_collection is used by GET method in collection handlers.