[ruby-core:18033] JRuby adding ConcurrencyError for fatal concurrent modification

In order to limit or reduce the likelihood that multiple threads 
concurrently performing unsynchronized modifications to a shared 
resource (like an Array, String, or Hash), we are adding a 
ConcurrencyError in JRuby. ConcurrentError extends ThreadError, and is 
primarily a debugging tool. The intended purpose is to prevent a Java 
stack trace from being generated, killing the thread in question, with 
no way to rescue and log it. This is not intended to be a 
generally-recoverable error; it's more to say "you were concurrently 
modifying something and shouldn't have been" so the code can be fixed.

I'm interested in comments on this. We tested all options for providing 
fully thread-safe core collections, and they all had pretty severe 
performance penalties involved. Given that Ruby 1.9 hopes to go to 
concurrent threads soon, I think we need to deal with this issue.

- Charlie