Change defaults for datastore_config

Rarely someone will set DATASTORE_CONFIG environment variable. It must be set not only in the interpreter environment but also in the run time environment of the engine/launcher, and also in the batch run time environnment (->resconfig).

Therefore, we should choose more conservative default settings - write to file and 100000 bytes threshold. It is conservative with regard to save space in the MongoDB database (in the fireworks and launches collections, and in gridfs). The docs should be adapted for this change.

Another aspect is to provide an option to facilitate gridfs cleanup. It is only in case one decides to remove models from a database so that all files in gridfs referenced only by deleted documents are also deleted.