[JENKINS-19453] Propose fix: mask interrupt while loading a class

I think the crux of the issue is that, in general, a failed class loading
will never get retried. It is certainly the case when another class loading has triggered a
recurisve class loading as in this test case.

In https://github.com/jenkinsci/remoting/pull/19 Jesse proposed to fix
this by dropping the whole RemoteClassLoader. But this will not work
since the said RemoteClassLoader cloud have loaded other classes that
might be already running. Dropping a classloader will not drop these
classes, and we end up just creating another classloader that loads
incompatible classes. That is a disaster waiting to happen.

In this fix, we simply make findClass non-interruptible. That is, if a
blocking remote operation gets interrupted, we catch that and simply
retry and refuse to give up. We'll remember to set the interrupt flag
back on, however, so that the interrupt signal doesn't disappear.

The net effect is as if the delivery of the interrupt was bit late than
usual. The way I see it, this is the only way to fix this.

The nesting of control structures here is horrible, but I can't think of
any better way to write this.