Allow different types of metadata to be stored in the data storage

[lni]

We currently assume that metadata is small and updated infrequently, e.g. membership info of each raft group is only updated when requested by the prophet. For such kind of metadata, we store its serialized form into data storage using the current interface.

There are other kind of metadata that we can't afford to dump the whole set to the data storage on each update, e.g. the client_id/update_id pairs used for preventing retried requests to be applied more than once, it is updated on each write. Such data has to be incrementally updated into the data storage and we have to consider how to GC its old data.

This issue only deals with how the data storage interface should be updated to support such new metadata types.