Report detected config servers clock skew in mongoS through some command

Currently only through mongos logs it's possible to know that your balancer doesn't work because of config servers clock skew. It will be helpful to have a way to know that through some command so that monitoring appliances like MMS could notify the user appropriately.

One option would be to add a new field to serverStatus that indicates if the last attempt of acquiring the distributed lock failed due to a clock skew on config servers.