conversation: Reverse messages in each log

While b7360ff1 took care of not
discarding new messges the reversion of the list resulted in reversed
entries for each log file (so the overall history was in the correct
order but each converstion was reversed).

We go this way around instead of using g_list_append() since the later
is slower.

Fix that by going through each log back to front.
1 job for reverse-log in 1 minute and 37 seconds (queued for 2 seconds)