Ensure we don't overflow the backoff value.

The first attempt to fix this (213102b36f890) did not correctly address
the issue.  The 32 bit signed integer frequently overflows, resulting in
a bad value for Random.nextInt().

The default for storm.messaging.netty.max_retries is now 30 (instead of
100), and there is an upper limit of 30 for max_retries.