Comment 2 for bug 1582044

It looks like this is intentional:

Preparing to unpack .../mysql-server-5.7_5.7.12-0ubuntu1_amd64.deb ...
Aborting downgrade from (at least) 10.0 to 5.7.
If are sure you want to downgrade to 5.7, remove the file
/var/lib/mysql/debian-*.flag and try installing again.

Also note this message repeated often in your logs, perhaps your network controller is unhappy:

[18317.930518] r8169 0000:01:00.0 enp1s0: rtl_counters_cond == 1 (loop: 1000, delay: 10).