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, 1 insertions, 1 deletions

diff --git a/docs/lifecycle.md b/docs/lifecycle.md
index 9fe9442..829e425 100644
--- a/docs/lifecycle.md
+++ b/docs/lifecycle.md
@@ -62,7 +62,7 @@ Then the node is either ready at once or joining:
 
 | Start | Without a remembered swarm | With one (state file) |
 |---|---|---|
-| `--genesis` | a new swarm, ready at once | that swarm again, empty; nodes of it still running merge with this one |
+| `--genesis` | a new swarm, ready at once | that swarm again, empty, from a new root, which nodes of it still running refuse (to join them, give seeds) |
 | document file holding a document | ready at once, on the document's swarm and epoch | the same; the two must name the same swarm |
 | `--recover-from` | ready at once, on the backup's swarm and epoch | the same; the two must name the same swarm |
 | seeds, or none | joining through the seeds | joining that swarm, through the seeds and the remembered members |