Fixed a deadlock in filterMissingDirectories.
Fixed a deadlock in filterMissingDirectories.

For #39.

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