aggregate: Avoid resetting ctime when an item md5 changes.

aggregate: Avoid resetting ctime when an item md5 changes.

Besides being wrong to do, this could lead to the wrong item
being expired, as follows: If B is added and at the same time
A is changed, then A's ctime may be set to the current time,
while B's is set to its creation time. Thus the new item, A,
is incorrectly removed as older.

(This interacted especially badly with the bug fixed by
90b4d079605b72bb50d1da41402d994960e10937.)