signatures: look for arrived changes only where the document grew

Received signatures of changes the document doesn't hold yet wait for
them, up to 65536 (MAX_PENDING). Each Signatures::offer, one per
received sync message, looked up every waiting one's change to find
those that came meanwhile.

The pending signatures now remember the document's heads at the last
look (Pending::scanned, Pending::arrived): while they stand, the
document holds nothing more, as it only grows, and offer looks up none.

0 files changed, 0 insertions, 0 deletions