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, 6 insertions, 0 deletions

diff --git a/docs/example.toml b/docs/example.toml
index d5af848..70d65b2 100644
--- a/docs/example.toml
+++ b/docs/example.toml
@@ -105,6 +105,12 @@ htpasswd = "/etc/continuum/htpasswd"
 # together at most.
 # key-logins-per-user = 100
 
+# The hash of the swarm's genesis change (hex, as its genesis node logs it and GET
+# /admin/epochs shows it): joining with an empty document, the only root this node
+# takes. Without, the one its peers name, none while they name different ones
+# (docs/access.md).
+# genesis-root = "…"
+
 # SSH keys, as authorized_keys lines (ssh-ed25519 <base64> [comment]), whose signature
 # on a strict remake's manifest this node takes: the values kept when the swarm turns
 # strict (docs/access.md). Without, it votes for no strict remake; a majority of the