Refactored ListingProxy.List.
Refactored ListingProxy.List.

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