Make use of directory generation numbers in fs.go.
Make use of directory generation numbers in fs.go.

GoogleCloudPlatform/gcsfuse,jacobsa/timeutil,kahing/gcsfuse,BanzaiMan/gcsfuse,jacobsa/ratelimit,GoogleCloudPlatform/gcsfuse,GoogleCloudPlatform/gcsfuse,mbrukman/gcsfuse,BanzaiMan/gcsfuse