Refactor backups metadata storage to use an external session.
Use a new backups for each API request (and close when finished).
Switch backups archive storage to gridfs.
Run -> RunTransaction.
NewBackupsStorage -> NewBackupStorage.
raw -> managed.
Remove superfluous interface tests.
envFileStorage -> backupBlobStorage.
Switch back to the more compact txn Ops formatting.
Drop the error return arg from newBackups.
Adjust to state.NewBackups.
Add a TODO about getting the machine ID.
Add DBOperator.RunTransaction.
Add DBOperator.Metadata.
c.Check -> c.Assert.
Add DBOperator.TxnOp.
Add DBOperator.BlobStorage.
Add DBOperator.AllMetadata.
Add DBOperator.RemoveMetadata (and Target -> metaColl).
Adjust some error messages.