Inserted the GMFs with the CacheInserter instead of the BulkInserter

See https://bugs.launchpad.net/oq-engine/+bug/1184624 for the motivation. Some performance tests on the cluster are to be performed.

EDIT: measurements on Cobain showed that the CacheInserter is 50% more efficient than the BulkInserter in the case of Miriam's island. It should be noticed that 2/3 of the time is spent in Python, when building the string to bulk insert from the Django objects. The time purely spent on the COPY FROM is 1/3 of the total, so the database is less stressed that one would expect. Still, the saving time is 6 times larger than the time to generate the build the GMF. The strategy of not saving the GMFs becomes every day more attractive...