Metasploit::Framework::Module::Cache#prefetch time

Improve the Metasploit::Framework::Module::Cache#prefetch by disabling uniqueness validators and checking for new records to eliminate queries involving `NULL`.  Timing statistics available at https://docs.google.com/spreadsheet/ccc?key=0Ah1hRHfpRW92dGxDRnN1ajJPWXlUWFBQNTdvWl9KZFE&usp=sharing.