Fix #24 - Spawn dedicated recv thread to solve LOR issues

Now, only one thread owns most of the contents of struct namenode. So,
we can safely manipulate that unlocked to avoid deadlock.

Tested with mt-hello example to follow.
Re: #24: Add hdfs_future_get_timeout API
Re: #24 - Add mt-hello example program

Spawns several threads, each of which loops to do several RPC calls.
Seems to work.