Disable fsync by default

A temporary test database does not need crash and power failure
recovery. Disabling this during the creation of the database (--nosync)
and during operation (-F) results in a speedup of about 10% on my Core2
system.

-F (fsync=no) has been available for a long time, but --nosync was
introduced in 9.3.

Tested with 9.4rc1