Tweak 'fat jar' handling to generally improve performance:

- Allow JarURLConnection to throw a static FileNotFoundException when
  loading classes. This exception is thrown many times when attempting
  to load a class and is silently swallowed so there is no point in
  providing the entry name.
- Expose JarFile.getJarEntryData(AsciiBytes) and store AsciiBytes in
  the JarURLConnection. Previously AsciiBytes were created, discarded
  then created again.
- Use EMPTY_JAR_URL for the JarURLConnection super constructor. The URL
  is never actually used so we can improve performance by using a
  constant.
- Extract JarEntryName for possible caching. The jar entry name
  extracted from the URL is now contained in an inner JarEntryName
  class. This could be cached if necessary (although currently it is
  not because no perceivable performance benefit was observed)

Fixes gh-1119