Export clients and related packages.
The main rule is to not import internal packages from exported packages.
The gateway client and related types are totally decoupled from the gateway
service (not shared types between the client and the server).
Instead the configstore and the runservice client currently share many types
that are now exported (decoupling them will require that a lot of types must be
duplicated and the need of functions to convert between them, this will be done
in future when the APIs will be declared as stable).
Just a raw replace of "github.com/pkg/errors".
Next steps will improve errors (like remote errors, api errors, not exist errors
etc...) to leverage its functionalities
rename the previous posix storage to posixflat and make it currently not user
selectable (since I'm not sure it's really worth using it).
The new posix storage uses the filesystem without any escaping so it's not a
real flat namespace.
This isn't a real issue since also minio is not a flat namespace and we are so
forced to use it like a hierarchycal filesystem.
`lts` was choosen to reflect a "long term storage" but currently it's just an
object storage implementation. So use this term and "ost" as its abbreviation
(to not clash with "os").
