gfid: introduce uuid based handles for inodes

gfid represents a gluter file id. This is a universally unique id
assigned to a logical inode, independent of the inode numbers
assigned by the various backend filesystems to that file/directory.
The gfid of a file/directory will be the same on servers depending
on the cluster translator in picture.

  The same gfid can be used as a handle across layers of various
translators and across servers and clients. This was not the case
previously as the cluster translators would pick the backend inode
number from one of the servers and convert that into a logical
inode number by performing some mathematical transforms.

  This new technique of addressing inodes also makes dynamic volume
management have a more robust implementation as the file handles
remain the same on all versions of the graphs, and allows for
seamless NFS daemon restarts as well.

  This change makes way for server originating communication which
was not possible earlier as the servers did not have any reliable
way of addressing client side inodes at all. gfid solves this problem
by preserving the same uuid as the handle on all the servers and
across all clients

Signed-off-by: Anand V. Avati <avati@blackhole.gluster.com>
Signed-off-by: Anand V. Avati <avati@amp.gluster.com>
Signed-off-by: Vijay Bellur <vijay@dev.gluster.com>

BUG: 971 (dynamic volume management)
URL: http://bugs.gluster.com/cgi-bin/bugzilla3/show_bug.cgi?id=971

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