esutils: cleanup and rewrite without xslt transformation for jaxb to json mapping

the textgrid-metadata xml to json for elasticsearch conversion was done quick and dirty and has lots of string to object and back transformation.

we should test the the following

• map from incoming jaxb to outgoing jackson object without xslt transformation
• instead of xslt use some simple stax parser for text extraction

this should lead to cleaner code and better performance