To prevent mutation of history, use merkle root hashes in addition to sequence numbers

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EDITED to include new discussion and work

`hyperdb` identifies node relationships using a feed key and a sequence number in that feed.

Because there is no hash related to the contents of the feed, it is possible for anybody who _holds the private key_ of a feed in a hyperdb to alter the content. (This would mean either a duplicitous publisher, or a compromised private key.) Old clients will refuse to sync, but new clients could be unable to tell the difference. A hyperdb version does not identify the contents or history of the database uniquely at all in this scenario.

To move towards solving this [`rootHashes` has been added to the hypercore api](https://github.com/mafintosh/hypercore/issues/142). Now, a single hash of all the root nodes in each feed should be added to each newly generated node, augmenting the use of sequence numbers and the vector clock([context](https://github.com/bnewbold/dat-deps/blob/dep-hyperdb/proposals/0000-hyperdb.md#reference-documentation), [explanation](https://github.com/bnewbold/dat-deps/blob/dep-multiwriter/proposals/0000-multiwriter.md#concepts-behavior-and-usage)). This should be verified when new nodes are read. The hash should be used when the `HyperDB.prototype.version` and `HyperDB.prototype.checkout` functions are used, to identify and verify versioned snapshots of the database.

Start with HyperDB.prototype.version and HyperDB.prototype.checkout, then review the linked hypercore rootHashes API discussion and the vector-clock references. Done means newly generated nodes carry a root hash, reads verify it, and versioned snapshots use it alongside sequence numbers to identify and verify history.

Written by the indexing model from the issue text.