Improve config.ini file readability and maintenance

Default config.ini should be empty, because obviously if there's nothing there then default values kicks in.
But to be clear about default values actually are, it's best to have it there, but commented out.
Currently I think we just show (some of?) default values as explicitly set but it makes maintenance a nightmare.

For example:

# the location of the chain shared memory files (absolute path or relative to application data dir)
shared-file-dir = "blockchain"

It should be:

### the location of the chain shared memory files (absolute path or relative to application data dir)
# shared-file-dir = "blockchain"

Then I could quickly tell all the non-default settings that were changed in such file but looking at non-commented out lines only.