Gets the instance marked as master

@param {Object} instances - Instances map

@returns {Object} master instance if one is found

{
id: instance_id,
instance: instance_data
}
function getMasterInstance(instances) {
        let instanceId;

        const instanceIds = Object.keys(instances);
        for (let i = 0; i < instanceIds.length; i++) {
            instanceId = instanceIds[i];
            if (instances[instanceId].isMaster) {
                return {
                    id: instanceId,
                    instance: instances[instanceId]
                };
            }
        }
        return null;
    }