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, 2 insertions, 0 deletions

diff --git a/src/simtest/timing.rs b/src/simtest/timing.rs
index 25dee02..5f93acc 100644
--- a/src/simtest/timing.rs
+++ b/src/simtest/timing.rs
@@ -282,6 +282,7 @@ async fn resent_hello_keeps_the_session() {
             ready: true,
             run_id: run,
             handshake,
+            root: None,
         })
     };
     for repeated in [handshake, handshake.saturating_sub(1)] {
@@ -325,6 +326,7 @@ async fn late_epoch_does_not_flip_a_peers_epoch() {
             ready: true,
             run_id: run,
             handshake: 1,
+            root: None,
         })
     };
     let status = |epoch: Epoch, epoch_seq: u32| {