Make ThreadContextStack EMPTY_STACK are real immutable empty stack.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1595901 13f79535-47bb-0310-9956-ffa450edef68
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git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1595904 13f79535-47bb-0310-9956-ffa450edef68
Javadoc.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1595905 13f79535-47bb-0310-9956-ffa450edef68