[WIP] replace "state name" with "workspace"

[mcanevet]

Will fix #18. I think we should clarify this before going further because it will probably break the API (at least the internal one).
I still have a lot of questions for you @cryptobioz and @raphink so I open this PR so that we can discuss about that in comments.

[mcanevet]

In pkg/client/client.go and internal/storage/storage.go, does ListStates retrieve all states or workspaces? If it's workspaces, then we should rename it to ListWorkspaces.

[mcanevet]

In internal/storage/storage.go, should the Name attribute of the State structure be Workspace?
Basically, shouldn't we have a Workspace structure with a States []*terraform.State and a Plans []*terraform.Plan attributes? Because we plan to store plans using Terraform v0.12's new show <plan> json output. Maybe it just a matter of renaming the StateCollection structure?

[mcanevet]

Hmmm... Actually, if we add a Workspace structure, we could reuse the State structure defined in github.com/hashicorp/terraform/terraform/state.go instead of duplicating most of its code. What do you think about that? Same with the Plan structure.

[mcanevet]

The more I read at the code, the more I think this is a huge change... But I still think we should do it.
I guess we should add this structure and refactor a lot of things:

type Workspace struct {
    LastModified time.Time `json:"last_modified"`
    Name         string    `json:"name"`
 
    // Keep Lock info
    Locked   bool     `json:"locked"`
    LockInfo LockInfo `json:"lock"`
 
    States []*terraform.State
    Plans  []*terraform.Plan
}

[mcanevet]

I added a new ListWorkspaces function. Maybe we should use this one instead of ListStates.