Make default log format less verbose

[lava]

Change the default format for logfiles to omit information about
the class, function and thread id. This information adds a lot
of visual noise and little value. In particular the thread id can
easily be confused with a unix time stamp.

Before:

5235076 vast DEBUG actor24 140049658796832 size_t,%20size_t,%20const%20vast.path&,%20vast.system.archive_type,%20caf.actor)<GLOBAL operator() disk_monitor.cpp:114 disk_monitor erasing removed ids from archive

After:

5235076 vast DEBUG actor24 disk_monitor.cpp:114 disk_monitor erasing removed ids from archive

[dominiklohmann]

Personally I like this change. I'd still like to see @tenzir/backend's opinion on this.

This needs a changelog entry.

[mavam]

I'm with @dominiklohmann. This is a refreshing change.

[dominiklohmann]

Just some changelog wordsmithing. Not sure about the second sentence, but please use the first one so it's using active over passive voice.

Approving already.