Added a flag, defaulting to 16 MiB.
Added a flag, defaulting to 16 MiB.

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