mstate: state package replacement using mgo.

mstate: state package replacement using mgo.

This is the first step towards replacing Zookeeper with MongoDB.
mstate will slowly implement the state API trying to preserve
the API as much as it makes sense.

This change implements preliminary support for Machines.

https://code.launchpad.net/~aramh/juju-core/mstate/+merge/111019

(do not edit description out of merge proposal)

[aram, 2012-06-19 13:59:02]

Please take a look.

[aram, 2012-06-19 14:06:02]

> On 2012/06/19 05:51:28, dfc wrote:
> > var mdoc = new(machineDoc) ?
> 
> my preference would be
> var doc machineDoc
> and use &doc below.

I like
  mdoc := new(machineDoc)
or
  var mdoc = new(machineDoc)

In any case, your proposal is certainly better than all the
&T{}s we have in our code now.

Do we replace all &T{}s we have in our code?

[aram, 2012-06-19 14:10:12]

Previous discussion: https://codereview.appspot.com/6304099/