In SERVER-23923 we discovered that the WriteConcernOptions attached to the OperationContext only report to have 'usedDefault' if the operation called 'extractWriteConcern'. This is not done for some requests which don't need to write, for example an OP_QUERY request. To aid users of WriteConcernOptions in understanding these subtleties, we proposed adding more states for WriteConcernOptions to be in. Something like 'kUninitialized', 'kUsedDefault', 'kExplicitlySet'.