libhdfs++: Implement parallel find with wildcards tool

The find tool will issue the GetListing namenode operation on a given directory, and filter the results using posix globbing library.

If the recursive option is selected, for each returned entry that is a directory the tool will issue another asynchronous call GetListing and repeat the result processing in a recursive fashion.

One implementation issue that needs to be addressed is the way how results are returned back to the user: we can either buffer the results and return them to the user in bulk, or we can return results continuously as they arrive. While buffering would be an easier solution, returning results as they arrive would be more beneficial to the user in terms of performance, since the result processing can start as soon as the first results arrive without any delay. In order to do that we need the user to use a loop to process arriving results, and we need to send a special message back to the user when the search is over.