Expose --io-threads, --worker-threads, and --no-dispatch run options
Minor perf. improvements for all web requests

This reduces byte[] creation by reusing ByteList's backing arrays from
JRuby directly instead of cloning them first. We can get away with
this because we don't modify them at all.

An extra exchange.dispatch(this) call was also removed. It's no longer
needed since the dispatch option was added in WunderBoss.