Lets operations working on all shards "join" and acquire
the same value of something, with that value being based on
whenever all shards reach the join.
Obvious use case: time stamp after one set of per-shard ops, but
before final ones.
The generation of the value is guaranteed to happen on the shards
that created the join point.
Based on the join-ops in CF::snapshot, but abstracted and made
caller responsibility. Primary use case is to help deal with
the join-problem of truncation.
Message-Id: <1456332856-23395-1-git-send-email-calle@scylladb.com>
