feat: Add timestamp to msgs_index7 and speed up Chatlist::try_load()

[iequidoo]

This speeds up chatlisting by 3--4 times on my biggest profile, so the difference is visually
noticeable, particularly after dropping disk caches. Before, msgs_index2 was used which has less
ordering and requires a full scan of msgs table.

explain query plan output for the "normal chatlist":

QUERY PLAN
|--SEARCH c USING INTEGER PRIMARY KEY (rowid>?)
|--SEARCH m USING INTEGER PRIMARY KEY (rowid=?) LEFT-JOIN
|--CORRELATED SCALAR SUBQUERY 1
|  |--MULTI-INDEX OR
|  |  |--INDEX 1
|  |  |  `--SEARCH msgs USING COVERING INDEX msgs_index7 (state=? AND hidden=? AND chat_id=?)
|  |  `--INDEX 2
|  |     `--SEARCH msgs USING COVERING INDEX msgs_index7 (state=? AND hidden=? AND chat_id=?)
|  `--USE TEMP B-TREE FOR ORDER BY
`--USE TEMP B-TREE FOR ORDER BY

Fix(?) #7848 . But even if not, this is an improvement anyway and i think that in general we should avoid full scans of msgs, it's the biggest table and otherwise the app doesn't scale well over time without DeleteDeviceAfter set.