Default worker childopts: GC logging, IPv4, -server; fixes #492

* Enable verbose GC tuning into {storm.home}/logs/gc-worker.log.N, capped at 100k
* added a new childopts interpolant, '%STORMHOME%', set to storm.home property at runtime.
* put example production childopts in the storm.yaml.example
* Added the 'preferIPv4Stack' setting, to make storm not bind to ipv6 interfaces by default