Make HDFS tool wrappers asynchronous.

The existing HDFS tool wrappers (src/hdfs/hdfs.hpp) are synchronous. They use os::shell to shell out the 'hadoop' commands. This makes it very hard to be reused at other locations in the code base.

The URI fetcher HDFS plugin will try to re-use the existing HDFS tool wrappers. In order to do that, we need to make it asynchronous first.