Cast fingerprint to int64 before we store it in db: fixes int-size
errors reported by cassandra.

It appears to me that int64(uint64_value) is bit-preserving. The
following two tests, inserted into StoreURLFetchResults, never produced
any output.

	var foo int64 = -1
	var bar uint64 = uint64(foo)
	if int64(bar) != foo {
		log4go.Error("PETE: Conversion error basic %d, %d", int64(bar), foo)
	}

	if uint64(int64(fr.FnvFingerprint)) != fr.FnvFingerprint {
		log4go.Error("PETE: Conversion error %d, %d", uint64(int64(fr.FnvFingerprint)), fr.FnvFingerprint)
	}