v2.4.2

add: client DNS cache
add: proxy is supported
add: listObjectsType2 interface
add: bucket lifecycle related interfaces
add: bucket policy related interface
add: bucket storage type interface
add: bucket CORS interface
add: bucket mirror back interface
add: bucket ACL interface
add: object tag interface
add: fetch object interface
change: unified exception error definition
fix: downloadFile defect
fix: GCC4.8 & ClientV1 compatibility