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

spring-websocket/src/test/resources/log4j.properties

@@ -1,9 +1,9 @@
 log4j.appender.console=org.apache.log4j.ConsoleAppender
 log4j.appender.console.layout=org.apache.log4j.PatternLayout
-log4j.appender.console.layout.ConversionPattern=%d{HH:mm:ss,SSS} [%c] - %m%n
+log4j.appender.console.layout.ConversionPattern=%d{HH:mm:ss,SSS} [%c][%t] - %m%n
 
 log4j.rootCategory=WARN, console
 log4j.logger.org.springframework.web=DEBUG
-log4j.logger.org.springframework.web.socket=DEBUG
+log4j.logger.org.springframework.web.socket=TRACE
 log4j.logger.org.springframework.messaging=DEBUG