Tagged checkpoints

[julio4]

Following #10

We can move the concept of tags to all kind of checkpoints, not only to barrier kind, and add checkpoints methods:

• is_tagged(&self, tag: &str)
• tag(&self) -> Option<&str>
• set_tag(&mut self, tag: Option<&str>)
• Display/Debug impls

I also suggest renaming all "named barrier" to only "tagged" for consistency.

We can also implement additional helpers in CheckpointExt to provide getter to part of history based on tagged checkpoints / barriers.

[namn-grg]

taking a shot at this