Synchronize request init in AbstractHttpSockJsSession

Although unlikely in practice (but not impossible), the SockJS
integration tests write a message while the request is initializing.
This change adds synchronization around request intiailization
for the SockJS HTTP sesion.

This is a backport of:
https://github.com/spring-projects/spring-framework/commit/59e02e63c427659e701cc371d210faf1a85c1d42

Issue: SPR-11916