This change adds a new manifest section to the checkpoint files.
The existing time moves into it, and we add to it the version of
the Pulumi CLI that created it, along with the names, types, and
versions of all plugins used to generate the file. There is a
magic cookie that we also use during verification.
This is to help keep us sane when debugging problems "in the wild,"
and I'm sure we will add more to it over time (checksum, etc).
For example, after an up, you can now see this in `pulumi stack`:
```
Current stack is demo:
Last updated at 2017-12-01 13:48:49.815740523 -0800 PST
Pulumi version v0.8.3-79-g1ab99ad
Plugin pulumi-provider-aws [resource] version v0.8.3-22-g4363e77
Plugin pulumi-langhost-nodejs [language] version v0.8.3-79-g77bb6b6
Checkpoint file is /Users/joeduffy/dev/code/src/github.com/pulumi/pulumi-aws/.pulumi/stacks/webserver/demo.json
```
This addresses pulumi/pulumi#628.
16ade183d8