[javaflow] More small changes to Continuation

[Kohsuke Kawaguchi <kk...@kohsuke.org>]

I'm setting up IDE with Cocoon, but meanwhile, more small changes.

1. replaced HashMap<Thread,Continuation> by ThreadLocal, as it does the 
job better (w/o synchronization). No point in marking static fields as 
transient.

2. I believe the current code doesn't run correctly if a thread running 
a continuation tries to run another continuation inside it. I fixed that.

3. added serialVersionUID. According to Java serialization experts at my 
work, any serializable class should have it. I assumed we aren't 
worrying about the backward compatibility right now, so I just set 1.

4. javadoc improvement on the continueWith method.

-- 
Kohsuke Kawaguchi