Problem
=======
- User does not set log-bin nor log-bin-index (or the relay-log
  counter parts).
- 5.7 server randomly picks a filename prefix - hostname ("H").
- 8.0 server picks a hardcoded filename prefix ("A").
- On upgrade, 8.0 server does not find A.index because it is named
  H.index.
- Conclusion: a different default value for index filename prefix in
  8.0 causes disruption later on.

Fix
===
Enable binary logging and binary log basename defaults to
"`hostname`-bin" name prefix if --log-bin is not configured,
then the `hostname`-bin.index is generated for binary log
index file name if --log-bin-index is not configured.
Relay log basename defaults to "`hostname`-relay-bin"
name prefix if --relay-log is not configured, then
the `hostname`-relay-bin.index is generated for relay log
index file name if --relay-log-index is not configured.
Then on upgrade, the 8.0 server can find the two index
files, since it is generating the same index file name
with the 5.7 serever.