Added synchronization to append() and copy() methods.

The source configuration is locked to avoid potential
concurrent modification exceptions while iterating over the key set.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1623861 13f79535-47bb-0310-9956-ffa450edef68
[CONFIGURATION-570] Alternative implementation of SystemConfiguration.getKeys().

In order to avoid ConcurrentModificationExceptions when iterating over the
keys of a SystemConfiguration, the iterator returned for the keys is now a
snapshot.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1623862 13f79535-47bb-0310-9956-ffa450edef68
[CONFIGURATION-570] Updated changes.xml.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1623863 13f79535-47bb-0310-9956-ffa450edef68