WIP: Change backup to be websockets-centric.


Note: Currently this patch is incomplete (mostly just test coverage).  My intention for the moment is to get feedback on the approach before investing more time.

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This is a slightly more focused version of #412 that also incorporates making the switch from POST-based requests to the websockets-based RPC.  In fact, this patch built out with that switch as the starting point rather than using the env-storage change as the starting point.  I also tried to build the APIs with an eye toward testability.

Key features:
* backup storage abstractions
* switch from file-oriented to storage oriented
* backup metadata
* addition of new backups collection in state
* switch from POST-based requests to websockets-based RPC
* addition of state API client method for backups

The file breakdown
----------------------
  Key files:
state/apiserver/client/backup.go -- (new) new API server method
state/api/backup.go -- (new) new API client method
state/backup/api/api.go -- (new) generic backup API stuff
state/backup/api/client.go -- (new) the underlying API client method implementation
state/backup/api/server.go -- (new) the underlying API server method implementation
state/backup/backup.go -- gutted and re-organized
state/backup/info.go -- (new) the backup metadata
state/backup/storage.go -- (new) the backup storage abstraction (and implementation)
state/storage.go -- (new) a more generic wrapper around the new backups MGO collection


  Secondary files:
state/backup/api/params.go
state/backup/sources.go

  Removals (cleanup):
state/apiserver/apiserver.go
state/apiserver/backup.go
state/apiserver/backup_test.go
state/apiserver/export_test.go

  Boilerplate:
state/apiserver/client/export_test.go
state/backup/api/export_test.go
state/backup/api/package_test.go
state/backup/export_test.go
state/backup/package_test.go
state/backup/api/export_test.go
state/backup/api/package_test.go
state/state.go

  Tests:
state/apiserver/client/backup_test.go
state/api/backup_test.go
state/backup/backup_test.go
state/backup/api/testing.go
Yeah, I don't think we want these URLs to be provider URLs. Backups will contain sensitive data, access should be restricted. (Ofc we'll need *internal* access to those URLs, at least until in-env storage lands, but this shouldn't be passed out of the state server.)