System level locking scheme for the gluster mount point

This merge changes GlusterFileSystem to use unix file locking (FileChannel) against a temp file to enforce a single thread access limit to the gluster mount. The lock is not global across the DFS.

The first two commits are for history, and a botched attempt at per-file-locking on the DFS. The third commit reverts to the original GlusterFileSystem.java, and the last implements the mount level locking.

Re-issue of this pull request with proper diff formatting: 

https://github.com/gluster/hadoop-glusterfs/pull/16