signatures: a transfer carries one signature per writer in it

Under a policy requiring signatures, a node takes a change only once a signature
vouches for it: one of the change itself, or of a later change of the same writer. A
writer is a node's run, each start, rejoin or drop making a fresh actor, so an epoch
gathers writers for as long as it lasts, and a transfer of its whole history (a joiner
pulling the document, a delta) must carry a signature for each of them, or the joiner
waits for the rest for good.

A message's signatures are now chosen by what it carries:

* changes, up to 256 signed among them: each one's signature;
* more (a whole document pulled): each writer's latest signed change among them, found
  by the document's change metadata (`Signatures::for_changes`), vouching for the rest;
* a delta: each writer with changes in it (`get_changes_meta(from)`), its latest
  verified signature (`for_delta`).

A message carries up to 65536 (6 MiB), and as many wait at the receiver for their
changes (a delta's come after it). So a transfer leaves writers unvouched only in an
epoch with more than 65536 runs writing since it began; a remake starts the count again.
Wire version 48.

2 files changed, 4 insertions, 3 deletions

diff --git a/src/transport/mod.rs b/src/transport/mod.rs
index cbb6734..fcdf56e 100644
--- a/src/transport/mod.rs
+++ b/src/transport/mod.rs
@@ -734,8 +734,9 @@ pub(crate) struct SyncEnvelope {
     /// to come (`swarm::hop`).
     pub(crate) sent: crate::clock::UnixMillis,
     /// Signatures of changes (`document::signatures`): of the changes the message
-    /// carries, or each actor's latest for a delta or past `signatures::MAX_PER_MESSAGE`
-    /// (`Signatures::for_changes`, `latest_each`), as `signatures::pack` packs them.
+    /// carries, each writer's latest among them past a few hundred; of a delta, each
+    /// writer's latest with changes in it (`Signatures::for_changes`, `for_delta`),
+    /// `signatures::MAX_PER_MESSAGE` at most, as `signatures::pack` packs them.
     pub(crate) signatures: serde_bytes::ByteBuf,
 }
 
diff --git a/src/transport/wire.rs b/src/transport/wire.rs
index 6784f8e..7fe8983 100644
--- a/src/transport/wire.rs
+++ b/src/transport/wire.rs
@@ -14,7 +14,7 @@ use super::{EncodedMessage, MessageKind, ParsedMessage};
 
 /// The wire format's version, the first byte of every payload. Bump it with every change
 /// to a message type or a codec (`golden_bytes` fails then).
-pub(crate) const VERSION: u8 = 47;
+pub(crate) const VERSION: u8 = 48;
 
 /// What the second byte of a payload says.
 fn tag(kind: MessageKind) -> u8 {