XMLPropertyListParser's getDocBuilder method need not be synchronized

[mailniranjan]

https://github.com/3breadt/dd-plist/blob/master/src/main/java/com/dd/plist/XMLPropertyListParser.java#L91 is a synchronized method and is causing performance problems in our peak load testing.

This method was synchronized from the first commit. (ad6ecc4#diff-2c27b48e0134dc885800b0487859238aR71). I can understand that there are initializing the docBuilderFactory variable lazily so that was the reason that method was synchronized.

With the current code, (https://github.com/3breadt/dd-plist/blob/master/src/main/java/com/dd/plist/XMLPropertyListParser.java#L49), the factory is initialized eagerly. So, the method need not be synchronized now.

[3breadt]

Thanks for your contribution.