Prior to this commit, if an instance of XStreamMarshaller was used
concurrently from multiple threads without having first invoked the
afterPropertiesSet() method, the private `xstream` field could be
initialized multiple times resulting in a ConcurrentModificationException
in XStream's internal DefaultConverterLookup.
This commit fixes these concurrency issues by making the `xstream` field
volatile and by implementing a double-checked locking algorithm in
getXStream() to avoid concurrent instance creation.
Closes gh-25017
ce11fdfa80