Add --fake parameter (bug 1061143)

Allow to mark migrations as run without really running them. Useful for database initialization.

We need this to the Landfill replacement.

See  https://bugzilla.mozilla.org/show_bug.cgi?id=1061143 for reference.

I've chosen this implementation because it allows to still use `--max` when using `--fake`.
schematic code is confusing, so let's try to make things clearer : add a comment explaining that you are setting the migration number in db without executing anything else.