Rollback to Savepoints

[tsutomi]

The journaled storage system (in development and testing) supports logging commands about resource modifications and data modifications.
At the present moment, anyway, it is not possible to ROLLBACK to a given commit (save-point), because this would mean reverting all the commands until that point, reconstructing the storage, by issuing the reverted commands.

The idea is to support

• the extension of the COMMIT and ROLLBACK to include a name of the work to commit or rollback to (eg. COMMIT 'save1' or ROLLBACK 'save1')
• revert logged commands to construct the current state of the database
• truncate all the modifications following the save point restored