More efficient _submessagesChanged

[edemaine]

Currently, creating a new message and updating a message incurs (via _submessagesChanged) a search for all messages within the same thread, to recompute submessageCount and submessageLastUpdate. But these quantities are easy to revise locally, incrementing count in the case of messageNew, and in both cases, maxing the root's current submessageLastUpdate with the updated message's date, both assuming the message is public. (When changing parents, and upon server restart, we should still use _submessagesChanged.)