Polish SockJsFrame
Add SockJsFrameType enum

SPR-10797
Support registering Filters in WebSocket test servers
Add SockJS client

This change adds a new implementation of WebSocketClient that can
connect to a SockJS server using one of the SockJS transports
"websocket", "xhr_streaming", or "xhr". From a client perspective
there is no implementation difference between "xhr_streaming" and
"xhr". Just keep receiving and when the response is complete,
start over. Other SockJS transports are browser specific
and therefore not relevant in Java ("eventsource", "htmlfile" or
iframe based variations).

The client loosely mimics the behavior of the JavaScript SockJS client.
First it sends an info request to find the server capabilities,
then it tries to connect with each configured transport, falling
back, or forcing a timeout and then falling back, until one of the
configured transports succeeds.

The WebSocketTransport can be configured with any Spring Framework
WebSocketClient implementation (currently JSR-356 or Jetty 9).

The XhrTransport currently has a RestTemplate-based and a Jetty
HttpClient-based implementations. To use those to simulate a large
number of users be sure to configure Jetty's HttpClient executor
and maxConnectionsPerDestination to high numbers. The same is true
for whichever underlying HTTP library is used with the RestTemplate
(e.g. maxConnPerRoute and maxConnTotal in Apache HttpComponents).

Issue: SPR-10797
Fix concurrency issues in SockJS session impls

This change ensures the server "WebSocketHandler" is notified of the
opening of a session before writing the open frame to the remote
handler. Any messages sent by the server "WebSocketHandler" while
getting notified of the opening get cached and flushed after the open
frame has been written.

This change introduces locking in AbtractHttpSockJsSession to guard
access to the HTTP response. The goal is to prevent contention between
client requests to receive messages (i.e. long polling) and
the application trying to write.

Issue: SPR-11916
Prevent unbounded retention of cancelled SockJS tasks

This change sets the removeOnCancelPolicy on the SockJS
ScheduledThreadPoolExecutor to true. This ensures that cancelled
tasks are removed immediately to avoid the "unbounded retention
of cancelled tasks" that is mentioned in the Javadoc of
ScheduledThreadPoolExecutor:

"By default, such a cancelled task is not automatically removed from
the work queue until its delay elapses. While this enables further
inspection and monitoring, it may also cause unbounded retention of
cancelled tasks. To avoid this, set setRemoveOnCancelPolicy to true,
which causes tasks to be immediately removed from the work queue at
time of cancellation."

Issue: SPR-11918
Use SimpleAsyncTaskExecutor in WebSocketClient impls

JettyWebsocketClient and StandardWebSocketClient now use the
SimpleAsyncTaskExecutor by default to ensure non-blocking connect.
Merge SockJS client-related changes