Support incremental update of outdated local cache

[Nikratio]

Currently, if the locally cached metadata is outdated, the entire database is downloaded from the backend.

We could make this a lot faster by storing the MD5 of each metadata object as object-metadata, and then only downloading objects whose MD5 differs from that of the locally cached block. For this, we probably want to implement a highly-parallel get_metadata(prefix) method for backend classes that uses multiple connections (multiplexed through async IO) to retrieve metadata for multiple objects.

The question is just: is it worth the complexity? How often do people have a partially outdated local cache.