Bug 56595: Also switch the cache in DateUtil.isADateFormat() to ThreadLocals to not have another syncpoint here. Again only very little data is kept, so no memory bloat should happen because of this.
Also do more simple checks before actually looking at the cache

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1647296 13f79535-47bb-0310-9956-ffa450edef68