Fix JSObject.equals

[Denis Lila]

Hi.

In this e-mail:
http://mail.openjdk.java.net/pipermail/distro-pkg-dev/2011-March/013329.html
I document the problems that JSObject.equals was causing
(i.e. breaking reflexivity and calling JSObject.toString()).
This patch fixes them. It does so by removing it
altogether. This means we no longer call PluginDebug.debug.

If anyone would miss it, I'm fine with putting it back
in, and just doing

public boolean equals(Object o) {
   PluginDebug.debug("equals called ", o);
   return this == o;
}

The other change is that with the new equals implementation,
we can use ConcurrentHashMaps in PluginObjectStore, which
allow us to remove synchronization from reads. An incomplete
but rigorous (though not necessarily correct) proof of why
ConcurrentHashMaps allow us to do that is here:
http://mail.openjdk.java.net/pipermail/distro-pkg-dev/2011-March/013360.html

Thank you,
Denis.
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