Poudriere builds failing, unable to rollback.

[George Hartzell]

I've been using Poudriere to build binaries for FreeBSD12.  Yesterday
I started seeing failures like this:

```
[...]
[00:44:25] [01] [00:00:00] Building archivers/arc | arc-5.21p
cannot rollback 'zroot/poudriere/jails/12_1-ports_and_hartzell-ref/01': there is a snapshot or bookmark more recent than 'zroot/poudriere/jails/12_1-ports_and_hartzell-ref/01 at prepkg'
[00:44:26] [01] [00:00:01] Error: Unable to rollback zroot/poudriere/jails/12_1-ports_and_hartzell-ref/01 to prepkg
=>> Error: Unable to rollback zroot/poudriere/jails/12_1-ports_and_hartzell-ref/01 to prepkg
[00:45:31] [02] [00:05:14] Finished devel/icu | icu-65.1,1: Success
[...]
```

It doesn't not always happen at the same point.

Not sure if it's related or not, but earlier in the day I wasn't
paying attention and ran of out space in that pool.

I don't see any mention of this in the Poudriere GitHub Issues.

Has anyone seen anything similar?  Any ideas what's going on?

Thanks,

g.