Enable internal proxy log by default to enable easier troubleshooting

[aviramha]

We have quite few issues that raise from layer saying internal proxy died, but users can't know why it happened.
I suggest the following:

1. Make internal proxy log by default to tempdir with mirrord_datetime_rand4chars.log
2. Prompt user to look into it and upload it to the issue they open when they get an internal proxy error.

#2742

[meowjesty]

Now that 1 has been done, I think 2 should be part of the error report, but do we also to add it in the "new issue" template thingy?