Implement configstore maintenance mode and export/import.
When configstore is set in maintenance mode it'll start only the maintenance and
export/import handlers.
Setting maintenance mode will set a key in etcd so all the configstore instances
will detect it and enter in maintenance mode. This is done asyncronously so it
could take some time (future improvements will add some api to show all the
configstore states)
Export is always available and will export the datamanager contents.
Import is available only during maintenance, given a datamanager export will
import it and reset etcd to this import state.
* Don't make cors enabled on all (*) by default.
* Handle related web.allowedOrigins options
* Only the gateway api should be called by a browser so setup the cors handler
only on it
Don't put datamanager base dirs inside the root of the ost but use a base path.
Let's do it now before releasing since this is a breaking change that requires
moving the ost data to the new path
* Rename to datamanager since it handles a complete "database" backed by an
objectstorage and etcd
* Don't write every single entry as a single file but group them in a single
file. In future improve this to split the data in multiple files of a max size.
`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").
Simone Gotti