implement config backup/rollback (snapshots)

[forkwhilefork]

Feature description

In Junos (and to a lesser extent IOS-XR), the following functionality is available:

• the last N commits are retained with the date and time they were made
• commits can have comments
• it is easy to diff between two previous commits
• it is easy to roll back N commits

We should be able to add all of this functionality.

Details

We can keep the files in a designated folder named by date/time. Settings for this (like how many commits are kept) can go in the main config file.

We need to store both the generated files and the config file. Rationale follows.

On one hand, the generated files are the "actual" config as executed by the system, so rolling back would be guaranteed to be the same. Theoretically, rolling back just the config file and then regenerating the generated files should be the same, but the tool might produce a different output from the same config file now. On the other hand, if we roll back only the generated files, we'd have to manually change the config file to match, or it would just get overwritten on the next regen.

If we keep both, we should prompt the user something to the effect of "this config produces a different output than it did at the time this snapshot was generated. here are the differences between the previous config and what it would be now. would you like to apply the new version, or the snapshot version?" This question should be asked before the rollback is actually executed.

Other

This is blocked by #43.

[forkwhilefork]

no longer blocked