Added fileSystem.ReleaseDirHandle.
Added fileSystem.ReleaseDirHandle.

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