Gate-ws

- add `WsService` method `GetConnection()` to get the connection
- fix `changelog` date error

- fix reconnect problem, add unexpected EOF capture

- add `io.ErrUnexpectedEOF` error capture, it caused v0.1.5 can't reconnect
- fix reconnect msg repeat add

- add `SpotUpdateAllDepthMsg` struct for parse all order book msg

- fix overlapping price for local order book example
- update README

- Support futures websocket.
- Modify channels name to with flag `Spot` or `Future`.
- Add field `TimestampInMilli` in models `SpotBalancesMsg`, `SpotFundingBalancesMsg`, `SpotMarginBalancesMsg`. Add
field `TimeInMilli` in model `SpotBookTickerMsg`
- Add new method `NewConnConfFromOption` to get a ConnConf flexible.
- Add new method `SubscribeWithOption` to support futures subscribe with id.
- Add example for both spot and futures connection use.
- Fix reconnect websocket failed bug.
- Optimizing code structure.