buid: Bipartite Unique Identifier
A BUID is a 128-bit unique ID composed of two 64-bit parts: shard and key. It is not only a unique ID, but also contains the sharding information, so that the messages with the same BUID could be stored together within the same DB shard. Also, when a message is stored in a shard, the shard part of the BUID can be trimmed off to save the space, and only the key part needs to be stored as the primary key. ...