close #39
Respect fiber's uncaught execution handler

Also, hint at instrumentation trouble (in the default
UncaughtExceptionHandler) if uncaught exception is NPE or CCE
Added FiberAsync.requestSync(long, TimeUnit)
Fix SuspendablesScanner bug

Handle the following case:

``` java
interface I1 {
   void foo();
}

interface I2 extends I1 {
}

class A implements I2 {
  @Suspendable void foo();
}
```

I2.foo() wasn’t being detected as suspendable. This fixes it.
Used a fixed thread-pool of size 1 as default group for fiber socket channels.
Add warning when JavaAgent isn't running
Rename channels.DelayedVal -> dataflow.Val
Added Var
Var
Var
Var
Fiber maintains AccessControlContext if a security manager is in use