LOG4J2-705 Fixed issue where Async Logger does not log thread context stack data. API change: added method getImmutableStackOrNull() to ThreadContext.ContextStack interface.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1609903 13f79535-47bb-0310-9956-ffa450edef68