Config server consistency check command

It would be good to have a convenient and foolproof way to check the consistency of the config servers in a sharded cluster.

Currently, the only indication that config servers are out of sync is the following message in the logs:

[mongosMain] ERROR: config servers not in sync! config servers config1.example.net:27019 and config3.example.net:27019 differ

The dbHash command can be used to compare the contents of config servers, but requires careful interpretation as some collections are expected to differ between config servers whereas others must not.

A command that could be run from a mongos to check config server consistency would be a good thing. Additionally, this command should report whether config servers are in 'read-only' mode (due to inconsistency or a config server being down), to give users confidence that the cluster meta-data is not updatable during critical operations such as cluster backup or restoration of a config server.