Refactored SyncFile and FlushFile.
Refactored SyncFile and FlushFile.

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