* add support group engine
* revert code
* add NewGroup function
* add engine group policy
* rename file name
* modify policy interface
* remove Init function from policy interface
* refactor Group Policy
* rename and comments
* rename and bug fix for WeightRoundRobinPolicy
* modify Slave function
* modify Slave function and add LeastConnPolicy
* use original Engine and Session
* remove unused count variables
* fix bug on NewEngineGroup
* remove unused method
* improve range and refactor
* add some comments and refactor
* implement GroupPolicy of GroupPolicyHandler
* refactor
* simple code
* add tests support for EngineGroup & fix some bugs
* improve the NewEngineGroup interface
* change the default policy of engine group
* fix some tests
Lunny Xiao