Performance enhancement for interface extension generation

I have found another performance enhancement: at these two places new instances of org.apache.xmlbeans.impl.config.Parser are created repeatedly for the same combination of Java source files and classpath.

https://github.com/apache/xmlbeans/blob/7dabbf1227023b3938f512632d5a750201ff2a3c/src/main/java/org/apache/xmlbeans/impl/config/BindingConfigImpl.java#L204

https://github.com/apache/xmlbeans/blob/7dabbf1227023b3938f512632d5a750201ff2a3c/src/main/java/org/apache/xmlbeans/impl/config/BindingConfigImpl.java#L216

When there are lots of interface extensions, the performance hit is quite significant.

See pull request https://github.com/apache/xmlbeans/pull/7