Custodian is responsible for the data stored in CouchDB databases.

Custodian scans the shards database, which details the location of
every shard of every database and ensures that operators are aware of
any shard that is under-replicated (has less than N copies).

Custodian accounts for data in transit (as indicated by the
mem3.redirects section) as well as nodes not recently known to be up.
