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.
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
Simone Gotti