Clarify the life cycle of ServerHttpExchange

[flowersinthesand]

The time where ServerHttpExchange is given means that the client sends a request header.

• read: reads the request body. #12.
• chunkAction: fired multiple times when a chunk from the request body is read. #14
• endAction: fired once when the request body is fully read - the end of the request. #14
• write: writes a chunk to the response body.
• end: completes the response - the end of the response.
• errorAction: fired multiple times if some error has occurred. #10
• closeAction: fired once when both request and response end normally or the underlying connection is aborted for some reason like error.

For now, close method should be renamed to end method.