allow to specify number of threads usable by netty for asynchronous io

Rationale:
1. Typically, tsdb can be collocated with hbase, but with default cpu * 2 number of workers and high input rate tsdb eats all cpus, leaving no room for hbase.
2. It can be too much cpus on box to give them all to tsdb (netty), so it is more wise to restrict number of tsdb workers to some reasonable value.