sync: a write between signing and making a message signed too

Review 4, F10: a sync message signed the node's latest change under the document's
write lock (`sign_sent`), then was made under its read lock: a write committed in
between went in the message unsigned, and under a policy requiring signatures waited at
the peer until the next signed change came with it, or a resend.

Now the message is made under a read lock taken after signing, and if a write came in
between (`AppState::sign_due` again under that lock), signed again: a few times at
most, past them it goes as before, its signature with the next message.

0 files changed, 0 insertions, 0 deletions