Transport: Raise bandwidth refiller thread priority

so I/O doesn't stall under high CPU load
- Raise DH generator thread priority to keep
  DH building out of event pumper thread
- Raise PRNG and YK generator thread priorites one notch
- Set I2PThread priority in constructor
Fixes problems mainly seen on Windows, which seems
to be much more sensitive to priority settings

core/java/src/gnu/crypto/prng/AsyncFortunaStandalone.java

@@ -56,7 +56,7 @@ public class AsyncFortunaStandalone extends FortunaStandalone implements Runnabl
         _isRunning = true;
         _refillThread = new I2PThread(this, "PRNG");
         _refillThread.setDaemon(true);
-        _refillThread.setPriority(Thread.MIN_PRIORITY+1);
+        _refillThread.setPriority(Thread.NORM_PRIORITY - 2);
         _refillThread.start();
     }