State upgrade step: Add EnvUUID to minUnits docs.

Add an upgrade step to add the environment uuid to minUnits docs. Update doc struct, call sites and tests.
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).