Avoid instantiating one StringBuilder per tag in the pom.xml

Instantiating an object takes only a small amount of time, yet we can
easily avoid the cumulative time by requiring only one instance of the
StringBuilder and reusing it all the time.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Release XML Handler after parsing the POM files

There is little use keeping the internal variables around that were
only required for parsing the POM files...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>