Added json validator to config.settings collection. Now, updates/inserts on config.settings with _id: "chunksize" will be rejected if the value is invalid. And any updates/inserts where the _id is an unexpected value will be rejected.
All existing incorrect values will be left as is.

Description of Linked Ticket

While investigating uses of NaN in the balancer, it came up that if the user updates the maxChunkSize to a value not accepted by the balancer (NaN, -10, etc.), the balancer and all other operations that rely on the balancer configuration setting will fail until the setting is changed. These operations are:

• all balancer operations
• add shard
• chunk autosplitting and initial chunk splitting
• manual migrations

This ticket is to investigate whether this is a problem and if there is a way to reject the write of an invalid value rather than checking it at runtime.