First version of multi-worker data loader.

This makes an initial implementation of the data loader with support for
multiple worker nodes (DM-1653). data-loader.py gets new option -W which
can be used multiple times to specify one or more worker host name.
Currently there is no way to specify different mysql port number for
different workers, all worker database servers are supposed to listen on
the same port. Mapping of chunks to hosts is currently done in a
round-robin mode, and of course different tables use the same chunk
mapping (see admin/chunkMapping module).
In the long term, mysql/zookeper db creation/removal should be atomic to avoid inconsistent state.