Vnode Vclock Implementation

[peschkaj]

Add vnode vclock flag in configuration to use the Riak 1.0 behavior of generating the vclock at the coordinating vnode

[peschkaj]

From Riak 1.0 release notes:

The way that Riak versions and updates objects has been
overhauled.  ClientIds are no longer used when updating objects, the
server handles all versioning using a vector clock id per-vnode.

New clusters are configured with the new vclock behavior turned on.
If you are performing a rolling upgrade of an existing cluster, once
all nodes have been upgraded the =app.config= needs to be updated to add
={vnode_vclocks, true}=.

Puts are now coordinated in the same way as on the original Dynamo
system.  Requests must be handled by a node in the preference list
(primary or fallback) for that bucket/key.   Nodes will automatically
forward to a valid node when necessary and increment the coord_redirs
stats counter.  The put is initially written to the local vnode before
forwarding to the remote vnodes.   This ensures that the updated vclock
for the riak object will replace the existing value or create siblings
in partitioning/failure scenario where the same client can see both
sides.