Parallel processing of messages results in bugs such as messages sent by
a new member being processed before the message that adds this member to
the chat, even when it has lower UID. In this case, messages are shown as
"[Unknown sender for this chat. See 'info' for more details.]", while
there is a message "Member ... added" right before, because writing the
information about new member to the database takes longer then reading
old index from the database.