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.
Use the go sql context functions (ExecContext, QueryContext etc...)
The context is saved inside Tx so the library users should only pass it one time
to the db.Do function.
* Override the provided remotesource id with the current one (it could not be
provided or provided with a different id but the remotesource ref is the way to
get the current remote source).
* When changing remotesource name check that a remote source with the new name
does not already exist.
All the validation must be done inside the configstore since it's the source of
truth.
The gateway could also do some validation to avoid bad requests to the
configstore when needed or when the logic resides outside the configstore (like
project setup or user registration)
RemoteRepositoryConfigType defines how a remote repository is configured and
managed. Currently only "remotesource" is supported.
In future other config types (like a fully manual config) could be supported.
`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").
* client: always parse the json error message field and return its contents
* Use ErrBadRequest and ErrNotFound in every handler and command
* Gateway: by default pass underlying service error (configstore, runservice) to
client keeping the status code and message. In future, if some errors must be
masked, we should change the specific parts that need special handling.
Simone Gotti