Some code cleanup

This pull request:

1. introduces a PerformanceMonitor.monitor decorator, as requested by Luigi
2. replaces the BulkInserter with the CacheInserter which is more efficient in terms
    of memory occupation (there were only a couple of occurrences of the BulkInserter left)
3. rename GmfAgg -> GmfData (this was missing from the renaming pull request https://github.com/gem/oq-engine/pull/1211);