In order to complete SERVER-23120 as quick as possible and unblock other work that depended on it, we installed a ShardRegistry on the config servers that works the same as the ShardRegistry running on shards or mongoses. This means then whenever it needs to reload information about the membership in the cluster, it makes a network connection to a config server (possibly this very same node), rather than just reading the contents of the config.shards collection from the local storage engine.

This should be cleaned up as part of the upcoming ShardRegistry refactoring work.