Ozone: ListBucket is too slow

Generated 3 million keys in ozone, and run listBucket command to get a list of buckets under a volume,

bin/hdfs oz -listBucket http://15oz1.fyre.ibm.com:9864/vol-0-15143 -user wwei

this call spent over 15 seconds to finish. The problem was caused by the inflexible structure of KSM DB. Right now ksm.db stores keys like following

/v1/b1
/v1/b1/k1
/v1/b1/k2
/v1/b1/k3
/v1/b2
/v1/b2/k1
/v1/b2/k2
/v1/b2/k3
/v1/b3
/v1/b4

keys are sorted in nature order so when we do list buckets under a volume e.g /v1, we need to seek to /v1 point and start to iterate and filter keys, this ends up with scanning all keys under volume /v1. The problem with this design is we don't have an efficient approach to locate all buckets without scanning the keys.