Made inode semantics much more precise.
Made inode semantics much more precise.

In particular, concurrent overwrites are now well-defined to look like
unlinks followed by re-links.

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