[icedtea-web][rfc] Fix for PR580: http://www.horaoficial.cl/ loads improperly

[Adam Domurad]

Hi all -- so this was a fix that I was working on for the unsigned 
applet confirmation pop-up dialogue (similar to Oracle's security levels).

Turns out this applet is a pretty a good stress test. Basically it 
causes 13 createApplet calls, which is a good way to reveal concurrency 
issues in getInstance.

Prior to patch:
  - Some or all of the 3 times that show up never finish loading, except 
after refresh
  After patch:
- All times show without needing to refresh, as expected

So why did it work mysteriously after refreshing ? After refresh, the 
shared classloader is still around (although it -should- be cleared! 
thats another bug!), and on the second round of loading all applets 
latch on to the correct classloader.

There was some basic synchronization on urlToLoader, but not enough. It 
needs to synchronize all instances of the unique key for the entire 
duration of the get/put.

The first approach I took was to simply make JNLPClassLoader#getInstance 
'synchronized' -- however parallel loading of different-unique-key 
applets would suffer. While same-unique-key applets must load 
sequentially for correct behaviour, a long-loading applet could make a 
short-loading applet wait a long time if getInstance was simply 
synchronized.

JNLPClassLoader continues to get more complicated, unfortunately ... but 
this fix is needed.
I have also removed a try ... catch that did nothing but re-threw the 
exception -- although I know this is unrelated, it allowed the diff to 
be cleaner :-) Feel free to force it back ...

Fix changelog:
2013-XX-XX  Adam Domurad  <adomurad at redhat.com>

     Fix PR580: http://www.horaoficial.cl/ loads improperly. Applets that
     must share a class-loader now load sequentially.
     * NEWS:
     Mention the fix.
     * netx/net/sourceforge/jnlp/runtime/JNLPClassLoader.java
     (getUniqueKeyLock): New, atomically grabs or creates a lock for the
     unique key.
     (getInstance): Ensure classloader initialization is locked by unique
     key.
     (decrementLoaderUseCount): Ensure classloader deinitialization is
     locked by unique key, get rid of no-longer used locks.
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