This change replaces the current 64 bit sequential GID with 64 bits
random bytes GID in an attempt to support persistent GID. Internally,
the GID is stored as uint64_t. For human representation and RPC
interface, GID is represented as 16 bytes hex string. For console
readout, 16 bytes are too long, so it is abbreviated to first 6 bytes.
When querying GID in RPC calls, user can speicfy the prefix of GID as
long as the prefix is shared by more than 1 GID entries.