Create SockJS ThreadPoolTaskScheduler extension

Commit 5d2e6f enabled the setRemoveOnCancelPolicy of the SockJS
ScheduledThreadPoolExecutor by default. However that property is only
available in JDK 1.7 or later. This change fixes the issue and
introduces an extension of ThreadPoolTaskScheduler for for use with
SockJS which is necessary in any case since ThreadPoolTaskScheduler
does not expose the underlying ScheduledThreadPoolExecutor otherwise.

Issue: SPR-11918