Avoid unnecessary DocumentBuilderFactory Creation #182

In DOMDifferenceEngine, we have:

    private DocumentBuilderFactory documentBuilderFactory =
        DocumentBuilderFactoryConfigurer.Default.configure(DocumentBuilderFactory.newInstance());

While if I provide my own DBF to DiffBuilder, above creation will be useless anyway due to override.

Since DOMDifferenceEngine is only used by DiffBuilder, I think it could be package private, and a constructor with DBF could be provided. (let DiffBuilder inject a DBF to it, either new or provided by caller).

Pretty much like the way to handle WhitespaceNormalizedSource.

I raise this issue because I observed some of my threads during LnP are running at below stack:

lang.Thread.State: RUNNABLE
at java.net.URLStreamHandler.parseURL(URLStreamHandler.java:255)
at java.net.URL.(URL.java:640)
at java.net.URL.(URL.java:508)
at sun.misc.URLClassPath$Loader.findResource(URLClassPath.java:701)
at sun.misc.URLClassPath.findResource(URLClassPath.java:225)
at java.net.URLClassLoader$2.run(URLClassLoader.java:572)
at java.net.URLClassLoader$2.run(URLClassLoader.java:570)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findResource(URLClassLoader.java:569)
at org.springframework.boot.loader.LaunchedURLClassLoader.findResource(LaunchedURLClassLoader.java:53)
at java.lang.ClassLoader.getResource(ClassLoader.java:1090)
at java.net.URLClassLoader.getResourceAsStream(URLClassLoader.java:233)
at org.apache.xerces.parsers.SecuritySupport$6.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at org.apache.xerces.parsers.SecuritySupport.getResourceAsStream(Unknown Source)
at org.apache.xerces.parsers.ObjectFactory.findJarServiceProvider(Unknown Source)
at org.apache.xerces.parsers.ObjectFactory.createObject(Unknown Source)
at org.apache.xerces.parsers.ObjectFactory.createObject(Unknown Source)
at org.apache.xerces.parsers.DOMParser.(Unknown Source)
at org.apache.xerces.parsers.DOMParser.(Unknown Source)
at org.apache.xerces.jaxp.DocumentBuilderImpl.(Unknown Source)
at org.apache.xerces.jaxp.DocumentBuilderImpl.(Unknown Source)
at org.apache.xerces.jaxp.DocumentBuilderFactoryImpl.setFeature(Unknown Source)

Which indicate DBF's creation is a heavy operation.
(I already have my own thread-local bases DBF passed into DiffBuilder)

Thanks in advance for looking into it.

Leon