Release v1.2.0

framework:

1. suported Kubernetes based service discovery, use k8s://namespace/service:port as the RPC Target value. More service discovery solutions can be supported by plugins.
2. logging arbitary value with json format, use logx.Infov/Errorv/Slowv.
3. disable stat logs by calling logx.DisableStat().
4. bug fixes and improvements.

goctl:

1. optimize code generation for mongodb without cache
2. optimize environment check before grpc code generation
3. optimize naming specification in model
4. print errors along with the goctl version and details
5. #915 resolve inline message code generation in grpc
6. #925, #929 optimize data type conversion for mysql in case NULL column
7. #968 read the destination path from symbol link