TIKA-1369 Resolve thread safety issue in ImageMetadataExtractor

Hi Chris,

This fix tries to resolve TIKA-1369 with handle thread safety by ThreadLocal and avoid other library dependencies.

I have run the test cases, so it seems correct to me, though I haven't found any other occurrence of ThreadLocal in Tika's source, so perhaps it's against your general patterns.

Regards,
Vilmos