Remove isStreaming flag from AbstractHttpSockJsSession

This change removes the need for the isStreaming field from the base
class AbstractHttpSockJsSession. This field was used to account for
differences between polling vs streaming SockJS sessions without having
to expose to sub-classes private fields that are otherwise protected
from concurrent access by the base class. The change manages to delegate
to sub-classes without providing direct access to protected fields.

Issue: SPR-12427

spring-messaging/src/main/java/org/springframework/messaging/simp/user/UserSessionRegistry.java

@@ -14,6 +14,8 @@ import java.util.Set;
  */
 public interface UserSessionRegistry {
 
+
+
 	/**
 	 * Return the active session id's for the given user.
 	 * @param user the user