add sync.Pool to manager JSON,XML,YAML,ProtoBuf,MsgPack Render object.
we can move JSON,XML,YAML,ProtoBuf,MsgPack Render implement to sub-module in Gin or other repository module based on this change.
`Gin` now have the `protobufBinding` function to check the request format, but didn't have a protobuf response function like `c.YAML()`.
In our company [ByteDance](http://bytedance.com/), the largest internet company using golang in China, we use `gin` to transfer __Protobuf__ instead of __Json__, we have to write some internal library to make some wrappers to achieve that, and the code is not elegant. So we really want such a feature.
