The JDK 7 signature of ConcurrentHashMap#keySet() returns a Set, while JDK 8 signature
returns a KeySetView, which is a new class in JDK 8. As a result, using Java 7 to run
an Openfire instance that is compiled with Java 8 (using the -source/-target 1.7
compilation flags) causes a java.lang.NoSuchMethodError.

To work around this problem, this commit applies a workaround as presented by
Martin Buchholz in https://bugs.openjdk.java.net/browse/JDK-8151366