Node level gluster mount lock.

Sorry about the mangled GlusterFileSystem.java diff-- apparently git doesn't appreciate copy and paste.

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.