Add support for incremental import (maybe?)

[lapo-luchini]

Right now the database is reset and imported from scratch each time; ideally I'd like to support incremental import, but that's not so easy, because no unique identifier is given for people and chats, other than the «current name» string.
Name changes could probably be identified by inspecting the first few messages in the chat, but since the dump seems to contain all the historical data I don't have a strong drive to attempt an incremental approach.