Write design document for system wide snapshots in Accumulo

Having the ability to snapshot an entire Accumulo instance and roll back to a previous snapshot seems very useful. The following are some possible use cases :

* Take a snapshot of Accumulo before upgrading.
* Take nightly snapshots of Accumulo. In the case of a catastrophic failure, roll back to the last snapshot.
* Copy a snapshot from one cluster to another, similar to export and import table but for the whole system. Could also possibly support copying the delta between two snapshots.

An issue does not seem like a good place for capturing a change of this scale, which is why the issue about writing a design document.

Start with the issue's proposed use cases and review Accumulo's existing table snapshot and export/import behavior. Produce a design document covering instance-wide snapshots, rollback, nightly or recovery workflows, cross-cluster copying, and possible snapshot deltas; completion means the design's scope, mechanisms, and open questions are documented for review.

Written by the indexing model from the issue text.