From https://github.com/zrepl/zrepl/issues/691
The last_n prune rule keeps everything, regardless of if it matches the
regex or not, if there are less than count snapshot. The expectation
would be to never keep non-regex snapshots, regardless of number.
package is now at 95% code coverage and the additional tests codify
all behavior specified in the docs
There is a slight change in behavior:
Intervals are now [duration) instead of (duration].
If the leftmost interval is not keep=all, the most recently created
snapshot will be destroyed if there are other snapshots within
that first interval.
Since we recommend keep=all all over the docs, and zrepl 0.3
will put holds on that snapshot if it is being replicated,
I feel like this is an acceptable change in behavior.
refs #292
fixup of 0bbe2befce
