roots: an epoch's root pinned, and named in every handshake

Review 2, N7, the care it raised: a joining node's empty document took
as the first epoch's root whatever dependency-less change came first,
from whichever peer it joined through; the first epoch's ID is all
zeros, so nothing pinned it. A remake's epoch has its ID, the snapshot
change's hash, which the rules hold its root to.

* Every node names its epoch's root in HELLO and HELLO_REPLY (the
  epoch's ID for a remake's, its genesis change for the first; wire
  version 45). A node whose document starts from another root of the
  same epoch refuses the handshake, logging it: another history under
  the swarm's ID, a forged root or a genesis started again.
* A joining node with an empty document takes as its root only the one
  it was given (--genesis-root, new), else the one its peers name, and
  none while they name different ones (access::screen::RootPin, kept
  in Screening across documents): one peer hands it another root only
  if it is all the joiner hears, or never, with --genesis-root. A root
  refused is listed with the refused, what builds on it with it.
* The genesis node logs the root (`The swarm's root`); GET
  /admin/epochs shows it (`root`).

A genesis started again under a remembered swarm ID used to merge with
the nodes of it still running; they now refuse it (another root), and
the docs say to join them with seeds instead.

The epoch's identity stays as it is (generation 0 all zeros): records
and epoch metadata name it so, and every comparison of epochs with
them.

1 files changed, 10 insertions, 0 deletions

diff --git a/docs/access.md b/docs/access.md
index c5365b7..782724a 100644
--- a/docs/access.md
+++ b/docs/access.md
@@ -318,6 +318,16 @@ policy's own change and everything after it, as every other node does. While nei
 its document nor what it receives holds a policy, it judges only that a root is the
 epoch's, and nothing costs more than applying the changes.
 
+An epoch's root is pinned. A remake's is its epoch's ID, the hash of its snapshot. The
+first epoch's is its genesis change, which its genesis node logs (`The swarm's root`)
+and `GET /admin/epochs` shows (`root`). Every node names its root in its handshakes: a
+node whose document starts from another root of the same epoch refuses the handshake
+(another history under the swarm's ID: a forged root, or a genesis started again).
+A node joining with an empty document takes as its root only the one it was given
+(`--genesis-root`), or else the one its peers name, and none while they name different
+ones. So one peer hands a joiner another root only if it is all the joiner hears, or
+never, with `--genesis-root`.
+
 What it doesn't do yet:
 
 * **Prove the user.** A change is its node's (with signatures required, proven) and its